t373: pin the production XDG-excludes seam GitWorktreesTest.seedingGitWorktrees builds

fleetd #362 review finding 2 protects GitWorktrees#previouslyEffectiveExcludesFileContent's
Java-side XDG_CONFIG_HOME/HOME read (it never goes through a git subprocess, so no
GIT_CONFIG_GLOBAL/GIT_CONFIG_SYSTEM isolation reaches it) with a gitEnv constructor seam. A
mutation run during the #372/#369 merge found that seam unpinned: stripping hermeticGitEnv(tmp)
from seedingGitWorktrees left every test green, poisoned XDG_CONFIG_HOME or not.

Adds seedingGitWorktreesResolvesTheExcludesFileFallbackInsideItsThrowawayDirectory, which asserts
the property directly (a GitWorktrees built for seeding resolves the fallback inside its own
throwaway directory) using a self-contained marker instead of relying on an externally poisoned
env var. Refactors hermeticGitEnv/seedingGitWorktrees into two-argument overloads (one taking an
explicit XDG_CONFIG_HOME / gitEnv) so the new test can pre-populate the marker before construction
while still going through the same production construction every other seeding test uses; no
behavior change for the 4 existing call sites.

fleetd/src/test/java/dev/ltms/fleet/session/GitWorktreesTest.java

@@ -1532,26 +1532,52 @@ class GitWorktreesTest {
      * for repo setup.
      *
      * <p>Scope, measured on the fleetd #369 merge and narrower than an earlier version of this
-     * comment claimed: this protects the 5 {@link #seedingGitWorktrees} sites plus — through
+     * comment claimed: this protects the {@link #seedingGitWorktrees} call sites plus — through
      * {@link #gitProcessBuilder} — every {@code git} subprocess the TEST itself starts. It does
-     * NOT cover the other 53 {@code new GitWorktrees(...)} constructions in this file, which pass
+     * NOT cover the {@code new GitWorktrees(...)} constructions elsewhere in this file that pass
      * no env override, so a production instance built that way still inherits the JVM's real
-     * environment. Stripping this override from {@code seedingGitWorktrees} leaves the class green
-     * both with and without the poison command above, so that half is currently unpinned.
+     * environment. (Re-measured for fleetd #373, on this file as it stands here: 4 call sites go
+     * through {@link #seedingGitWorktrees(Path, String, Path)} — not 5, an earlier count this
+     * comment and fleetd #373's own ticket text both repeated without re-running it — out of 59
+     * total {@code new GitWorktrees(...)} occurrences, one of which is the shared construction
+     * inside {@link #seedingGitWorktrees(Path, String, Map)} itself. This class-wide count moves
+     * every time a test is added, so treat any number here as a snapshot, not a fact to cite
+     * without recounting.) Stripping the {@code gitEnv} override from a {@link
+     * #seedingGitWorktrees} call site leaves the class green both with and without the poison
+     * command above for that call site's OWN test, so that half was unpinned until fleetd #373
+     * added {@link #seedingGitWorktreesResolvesTheExcludesFileFallbackInsideItsThrowawayDirectory}
+     * below, which asserts the property directly instead of relying on a poisoned real machine.
      */
     private static Map<String, String> hermeticGitEnv(Path tmp) {
+        return hermeticGitEnvAt(tmp.resolve("hermetic-xdg-config-home-" + System.nanoTime()));
+    }
+
+    /** Same isolation as {@link #hermeticGitEnv(Path)}, with an explicit {@code XDG_CONFIG_HOME}
+     *  instead of a fresh nanoTime-unique one under {@code tmp} — used by
+     *  {@link #seedingGitWorktreesResolvesTheExcludesFileFallbackInsideItsThrowawayDirectory}
+     *  (fleetd #373) so it can pre-populate that directory with a marker BEFORE the production
+     *  {@link GitWorktrees} instance reads it, something the random per-call name from
+     *  {@link #hermeticGitEnv(Path)} makes impossible to predict from outside. */
+    private static Map<String, String> hermeticGitEnvAt(Path xdgConfigHome) {
         return Map.of(
                 "GIT_CONFIG_GLOBAL", "/dev/null",
                 "GIT_CONFIG_SYSTEM", "/dev/null",
                 "GIT_TERMINAL_PROMPT", "0",
-                "XDG_CONFIG_HOME", tmp.resolve("hermetic-xdg-config-home-" + System.nanoTime()).toString());
+                "XDG_CONFIG_HOME", xdgConfigHome.toString());
     }
 
     /** {@link GitWorktrees}'s full test seam, with a {@code memberSkillsSource} and no other
      *  overrides — the shape every seeding test below needs, isolated via {@link #hermeticGitEnv}. */
     private static GitWorktrees seedingGitWorktrees(Path root, String memberSkillsSource, Path tmp) {
-        return new GitWorktrees(root.toString(), null, _ -> {}, null, null, memberSkillsSource,
-                hermeticGitEnv(tmp));
+        return seedingGitWorktrees(root, memberSkillsSource, hermeticGitEnv(tmp));
+    }
+
+    /** Same shape as {@link #seedingGitWorktrees(Path, String, Path)}, taking an already-built
+     *  {@code gitEnv} directly rather than computing one via {@link #hermeticGitEnv(Path)} — lets
+     *  fleetd #373's test drive the exact production construction a real member spawn uses, with a
+     *  {@code gitEnv} it has already pre-populated a marker into. */
+    private static GitWorktrees seedingGitWorktrees(Path root, String memberSkillsSource, Map<String, String> gitEnv) {
+        return new GitWorktrees(root.toString(), null, _ -> {}, null, null, memberSkillsSource, gitEnv);
     }
 
     /** Acceptance criterion 2 (part 1): a worktree with no {@code .claude/} at all gets the skill
@@ -1784,6 +1810,63 @@ class GitWorktreesTest {
                         + "after skill seeding ran — got:\n" + porcelain);
     }
 
+    /**
+     * fleetd #373. Pins the production seam that fleetd #362 review finding 2 protects: {@link
+     * GitWorktrees#previouslyEffectiveExcludesFileContent}'s XDG-fallback branch reads {@code
+     * XDG_CONFIG_HOME}/{@code HOME} straight in Java, not through a {@code git} subprocess, so
+     * {@code gitEnv} — the constructor seam every {@link #seedingGitWorktrees} instance in this
+     * class is built with — is the ONLY thing that can isolate it. A mutation run during the
+     * fleetd #372/#369 merge found this unpinned: replacing {@code hermeticGitEnv(tmp)} with
+     * {@code null} in {@link #seedingGitWorktrees(Path, String, Path)} left every test in this
+     * class green — the 56 tests that would fail against a real machine's poisoned {@code
+     * XDG_CONFIG_HOME} were fixed by fleetd #369's subprocess-level isolation, but none of them
+     * looks at what THIS Java-side read resolves, so deleting the override stays invisible.
+     *
+     * <p>This test asserts the PROPERTY, not the constructor argument: a {@link GitWorktrees}
+     * built for seeding — through the very same {@link #seedingGitWorktrees(Path, String, Map)}
+     * construction every other seeding test in this class goes through — must resolve the
+     * excludes-file fallback inside its own throwaway {@code gitEnv}-supplied directory. It needs
+     * NO externally-set poisoned environment variable: the marker pattern below is written ONLY
+     * inside a throwaway {@code XDG_CONFIG_HOME} this test controls directly (bypassing {@link
+     * #hermeticGitEnv(Path)}'s unpredictable nanoTime-named directory, via {@link
+     * #hermeticGitEnvAt}, so the marker can be in place before the production instance ever reads
+     * it), reachable ONLY through the {@code gitEnv} seam. If that seam is stripped, the
+     * production code instead falls back to resolving the REAL {@code XDG_CONFIG_HOME}/{@code
+     * HOME} of the machine running the test — which does not carry this marker — so the marker
+     * file below shows up as untracked and the assertion fails on any machine, with no poison
+     * command required. See the PR body for the pasted failure from actually running that
+     * mutation (removing the {@code gitEnv} override from this test's own construction).
+     */
+    @Test
+    void seedingGitWorktreesResolvesTheExcludesFileFallbackInsideItsThrowawayDirectory(@TempDir Path tmp)
+            throws Exception {
+        Path xdgConfigHome = tmp.resolve("cb373-xdg-config-home");
+        Files.createDirectories(xdgConfigHome.resolve("git"));
+        Files.writeString(xdgConfigHome.resolve("git").resolve("ignore"), "cb373-xdg-fallback-marker\n");
+        Map<String, String> gitEnv = hermeticGitEnvAt(xdgConfigHome);
+
+        Path repo = initRepo(tmp.resolve("repo"));
+        Path skillsSource = tmp.resolve("skills-src");
+        writeSkill(skillsSource, "implementer", "IMPLEMENTER SKILL\n");
+        GitWorktrees seeding = seedingGitWorktrees(tmp.resolve("wts"), skillsSource.toString(), gitEnv);
+
+        String wt = seeding.add(repo.toString(), "cb-373-xdg-seam", "HEAD");
+        assertEquals("IMPLEMENTER SKILL\n",
+                Files.readString(Path.of(wt, ".claude", "skills", "implementer", "SKILL.md")),
+                "fixture check — the skill really was seeded, so previouslyEffectiveExcludesFileContent ran");
+
+        Files.writeString(Path.of(wt, "cb373-xdg-fallback-marker"),
+                "would only be invisible to git status if the fallback resolved THIS throwaway "
+                        + "XDG_CONFIG_HOME rather than the real machine's\n");
+
+        String porcelain = fullStatus(Path.of(wt));
+        assertEquals("", porcelain,
+                "the marker pattern lives only in this test's throwaway XDG_CONFIG_HOME; git "
+                        + "status must still be empty, proving the production seam resolved the "
+                        + "excludes-file fallback through the gitEnv seam rather than the JVM's "
+                        + "real environment — got:\n" + porcelain);
+    }
+
     /**
      * fleetd #369, acceptance criterion 4 — make the fix hard to undo by accident. Every git
      * subprocess this class starts is required to go through {@link #gitProcessBuilder}, the one