fleetd #439: pin the caller at the fleet_list call site, not just the gate

Review of PR #462 found M2: the coordinatorVisibleTo gate (then an inline
principal(exchange).isPrimary() check) could survive a mutation that
replaced the argument with a literal true at the one production call
site, because every existing test drove listFleet directly and supplied
the boolean itself -- nothing exercised the handler's own call.

- Name the decision: FleetMcp.coordinatorVisibleTo(Principal), a small
  package-private predicate next to denyFor/recordPrimarySingleton. The
  fleet_list handler now calls coordinatorVisibleTo(principal(exchange))
  instead of inlining .isPrimary().
- Pin the predicate's role table in FleetMcpAuthzTest
  (onlyThePrimaryMaySeeTheCoordinatorRow), covering primary/worker/
  architect and, newly, anonymous.
- Add a source-reading detector at the boundary
  (theFleetListHandlerActuallyConsultsCoordinatorVisibleTo), same idiom as
  toolsTheServerRegisters/everyRegisteredToolHasItsHandlerActionPinned: it
  reads FleetMcp.java, isolates the listHandler block, asserts (as a
  control) that the block actually contains a listFleet( call, then
  asserts the call's trailing boolean argument is exactly
  coordinatorVisibleTo(principal(exchange)) -- not a literal true/false.

Both mutations from the review were reproduced and killed by these tests,
then reverted; see the PR body for the full break-and-restore transcript.

fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java

@@ -427,7 +427,7 @@ public final class FleetMcp {
                             leadSeats, callers == null ? Map.of() : callers.leads(),
                             callerTerminal(exchange),
                             new CoordinationSource(leadChannel, peers),
-                            principal(exchange).isPrimary());
+                            coordinatorVisibleTo(principal(exchange)));
                 };
         BiFunction<McpSyncServerExchange, McpSchema.CallToolRequest, McpSchema.CallToolResult> stopHandler =
                 (exchange, req) -> {
@@ -580,6 +580,20 @@ public final class FleetMcp {
         }
     }
 
+    /**
+     * fleetd #439: only the primary may read {@code fleet_list}'s {@code coordinator} row —
+     * lead-to-lead coordination state (coord-ids, mailbox facts, held-message previews), never the
+     * roster. Split out of the {@code fleet_list} handler, same reason as {@link #denyFor} and
+     * {@link #recordPrimarySingleton}: the decision must be unit-testable without fabricating an
+     * SDK {@code McpSyncServerExchange}, and the handler must call this named predicate rather than
+     * inlining the check, so a future edit cannot silently pass a literal instead of asking who
+     * called ({@code FleetMcpAuthzTest.theFleetListHandlerActuallyConsultsCoordinatorVisibleTo}
+     * reads the source and asserts the handler calls this method by name, not a literal).
+     */
+    static boolean coordinatorVisibleTo(Principal caller) {
+        return caller.isPrimary();
+    }
+
     /** The worker identity resolved from this call's connection, or {@code null} if the primary. */
     private static String callerTerminal(McpSyncServerExchange exchange) {
         Object v = exchange.transportContext().get(CALLER_TERMINAL);

fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpAuthzTest.java

@@ -187,6 +187,71 @@ class FleetMcpAuthzTest {
                 "no CallerResolver supplied ⇒ authorization not enforced (legacy behaviour)");
     }
 
+    // --- fleetd #439: who may see fleet_list's coordinator row ----------------------------------
+
+    /**
+     * fleetd #439: {@link FleetMcp#coordinatorVisibleTo} is the whole policy decision for
+     * {@code fleet_list}'s {@code coordinator} row — lead-to-lead coordination state, not roster
+     * observation. Only the primary may see it; a worker, an architect, and (the case the previous
+     * pass of this ticket did not cover) an anonymous caller must all be refused.
+     */
+    @Test
+    void onlyThePrimaryMaySeeTheCoordinatorRow() {
+        assertTrue(FleetMcp.coordinatorVisibleTo(PRIMARY), "the primary must see its own coordination state");
+        assertFalse(FleetMcp.coordinatorVisibleTo(WORKER_A), "a worker must not see lead-to-lead coordination state");
+        assertFalse(FleetMcp.coordinatorVisibleTo(ARCH_DESIGN),
+                "an architect holds READ today, but that must not extend to coordinator");
+        assertFalse(FleetMcp.coordinatorVisibleTo(ANON), "authenticated as nothing must not see it either");
+    }
+
+    /**
+     * fleetd #439 / PR #462 review finding M2: the predicate above can be perfectly correct while
+     * the one production call site (the {@code fleet_list} MCP handler) never actually asks it —
+     * a literal {@code true} compiles, and the whole suite stayed green under that mutation because
+     * every existing test drives {@link FleetMcp#listFleet} directly and supplies the boolean
+     * itself. This test reads {@code FleetMcp.java}'s own source (same idiom as {@link
+     * #toolsTheServerRegisters()} / {@link #everyRegisteredToolHasItsHandlerActionPinned()}) and
+     * asserts the handler's call passes {@code coordinatorVisibleTo(principal(exchange))} — not a
+     * literal {@code true} or {@code false} — as {@code listFleet}'s trailing argument.
+     *
+     * <p>Anchored on argument position, not a bare substring search: {@code true} appears many
+     * times elsewhere in this file for unrelated reasons, so a plain {@code contains("true")}
+     * check would prove nothing. The pattern requires the literal text immediately before the
+     * closing {@code );} of the {@code listFleet(} call inside the handler block to be exactly
+     * {@code coordinatorVisibleTo(principal(exchange))}.
+     */
+    @Test
+    void theFleetListHandlerActuallyConsultsCoordinatorVisibleTo() throws Exception {
+        String source = Files.readString(MCP_SOURCE);
+
+        // Isolate the fleet_list handler block: from its declaration up to the next handler's
+        // declaration. A change to variable naming would break this scrape loudly (see the control
+        // assertion just below), rather than silently reporting "no violation found".
+        int start = source.indexOf("listHandler =");
+        assertTrue(start >= 0, "could not find the fleet_list handler (listHandler) in " + MCP_SOURCE
+                + " -- the scrape has stopped matching, fix the anchor before trusting this test");
+        int end = source.indexOf("stopHandler =", start);
+        assertTrue(end > start, "could not find the handler declared after listHandler to bound the scrape");
+        String handlerBlock = source.substring(start, end);
+
+        // CONTROL: the block we scraped really does contain a call to listFleet(...) -- if this
+        // fails, the anchors above moved and the assertion below would otherwise pass on nothing.
+        assertTrue(handlerBlock.contains("listFleet("),
+                "control failed: the scraped listHandler block contains no listFleet( call at all -- "
+                        + "the anchors have drifted, this test is not testing what it claims to");
+
+        Pattern trailingArg = Pattern.compile(
+                "listFleet\\([^;]*?,\\s*(coordinatorVisibleTo\\(principal\\(exchange\\)\\)|true|false)\\s*\\)\\s*;",
+                Pattern.DOTALL);
+        Matcher m = trailingArg.matcher(handlerBlock);
+        assertTrue(m.find(), "could not locate listFleet(...)'s trailing boolean argument in the "
+                + "listHandler block -- the call shape changed, update this test's anchor: " + handlerBlock);
+        String trailing = m.group(1);
+        assertEquals("coordinatorVisibleTo(principal(exchange))", trailing,
+                "the fleet_list handler must ask coordinatorVisibleTo(principal(exchange)) who is "
+                        + "calling, not pass a literal boolean -- found: " + trailing);
+    }
+
     // --- which action each tool hands the gate (fleetd #272) ------------------------------------
 
     /**