fleetd #736: release clears the member's presence entry

SessionManager.releaseRemoved() tore down a member's registry row and pane
but never cleared it from MemberPresence, so a terminal stayed marked
"present" for the daemon's lifetime after release/idle-reap/shutdown drain.
Clear it in the method's unconditional finally block, alongside the other
must-always-run teardown step, so every release path (explicit release,
the idle reaper's releaseIfCurrent, and a shutdown drain) forgets it the
same way, and a throw from the dirty-worktree check does not skip it.

MemberPresence.forget(null) throws NullPointerException (verified empirically:
ConcurrentHashMap.remove(null) NPEs on key.hashCode()), so the new call guards
on a non-null, non-blank terminal id rather than relying on forget to no-op.

fleetd/src/main/java/dev/ltms/fleet/session/SessionManager.java

@@ -470,6 +470,12 @@ public final class SessionManager implements TurnListener {
                 MemberSession resolved = resolveAgentSessionId(removed, removedHandle);
                 notifyReleased(new ReleaseDetail(resolved.terminalId(), resolved.worktree(),
                         resolved.branch(), snapshotRef, resolved.agentSessionId()));
+                String terminal = removed.terminalId();
+                if (terminal != null && !terminal.isBlank()) {
+                    // Without this, a terminal stays marked present after its pane is gone, so a
+                    // later send to the same id would read as deliverable instead of refused.
+                    presence.forget(terminal);
+                }
             }
         }
         // CB-581: the pane must always stop, even if the dirty check above threw. A session removed

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

@@ -1452,6 +1452,105 @@ class SessionManagerTest {
                         + "dirty check threw");
     }
 
+    // --- fleetd #736: a release must forget the member's presence entry, not just its registry
+    // row ---------------------------------------------------------------------------------------
+
+    @Test
+    void releaseByPaneIdForgetsThePresenceEntry() {
+        FakeHerdr herdr = new FakeHerdr();
+        SessionManager sessions = sessionManager(herdr);
+        MemberSession session = sessions.acquire("ltms-local", null, "/caller", "term_primary");
+        String terminal = session.terminalId();
+        sessions.asPresence().markPresent(terminal);
+        assertTrue(sessions.asPresence().isPresent(terminal), "present before the release");
+
+        sessions.release(session.paneId());
+
+        assertFalse(sessions.asPresence().isPresent(terminal),
+                "release must forget the terminal's presence, not just remove its registry row");
+    }
+
+    @Test
+    void reapIdleForgetsThePresenceEntryToo() {
+        long[] clock = {0};
+        FakeHerdr herdr = new FakeHerdr();
+        SessionManager sessions = sessionManager(herdr, () -> clock[0]);
+        MemberSession session = sessions.acquire("ltms-local", null, "/caller", "term_primary");
+        String terminal = session.terminalId();
+        sessions.asPresence().markPresent(terminal);
+        assertTrue(sessions.asPresence().isPresent(terminal), "present before the reap");
+
+        clock[0] = 11;
+        assertEquals(1, sessions.reapIdle(10), "READY session past TTL is reaped");
+
+        assertFalse(sessions.asPresence().isPresent(terminal),
+                "the idle-reap release path (releaseIfCurrent) goes through the same teardown "
+                        + "funnel as an explicit release, so it must forget presence too");
+    }
+
+    @Test
+    void shutdownDrainAlsoForgetsThePresenceEntry() {
+        FakeHerdr herdr = new FakeHerdr();
+        SessionManager sessions = sessionManager(herdr);
+        MemberSession session = sessions.acquire("ltms-local", null, "/caller", "term_primary");
+        String terminal = session.terminalId();
+        sessions.asPresence().markPresent(terminal);
+        assertTrue(sessions.asPresence().isPresent(terminal), "present before the drain");
+
+        sessions.drainAll(TimeUnit.MILLISECONDS.toNanos(100));
+
+        assertFalse(sessions.asPresence().isPresent(terminal),
+                "a shutdown drain still ends the member's process, so presence must be cleared "
+                        + "exactly as it is for any other release cause");
+    }
+
+    @Test
+    void releaseOfAnUnknownPaneIdDoesNotThrow() {
+        FakeHerdr herdr = new FakeHerdr();
+        SessionManager sessions = sessionManager(herdr);
+
+        assertDoesNotThrow(() -> sessions.release("no-such-pane"),
+                "releasing a pane id that was never registered must be a no-op, not a throw");
+    }
+
+    @Test
+    void releaseStillForgetsPresenceWhenDirtyCheckThrows() {
+        FakeHerdr herdr = new FakeHerdr();
+        RecordingWorktrees worktrees = new RecordingWorktrees();
+        SessionManager sessions = sessionManager(herdr, worktrees);
+        MemberSession s = sessions.acquire("ltms-local", null, "/caller/proj", null,
+                new WorktreeRequest("fleetd-736", null));
+        String terminal = s.terminalId();
+        sessions.asPresence().markPresent(terminal);
+        worktrees.failHasUncommittedWith(new WorktreeException("git status exited 128"));
+
+        assertDoesNotThrow(() -> sessions.release(s.paneId()),
+                "a throwing dirty check must not abort the release");
+
+        assertFalse(sessions.asPresence().isPresent(terminal),
+                "presence must be forgotten even when the dirty check throws, which pins the "
+                        + "forget call to the finally block that runs no matter what happened above");
+    }
+
+    @Test
+    void releaseLeavesADifferentStillLiveMembersPresenceUntouched() {
+        FakeHerdr herdr = new FakeHerdr();
+        SessionManager sessions = sessionManager(herdr);
+        MemberSession released = sessions.acquire("ltms-local", null, "/caller/a", "ownerA");
+        MemberSession stillLive = sessions.acquire("ltms-local", null, "/caller/b", "ownerB");
+        sessions.asPresence().markPresent(released.terminalId());
+        sessions.asPresence().markPresent(stillLive.terminalId());
+        assertTrue(sessions.asPresence().isPresent(stillLive.terminalId()),
+                "present before the release of the other member");
+
+        sessions.release(released.paneId());
+
+        assertFalse(sessions.asPresence().isPresent(released.terminalId()),
+                "the released terminal is forgotten");
+        assertTrue(sessions.asPresence().isPresent(stillLive.terminalId()),
+                "a still-live member's presence must survive an unrelated release");
+    }
+
     // --- fleetd #316: the dirty check must be re-taken after the worker is stopped, not trusted
     // stale from before it ------------------------------------------------------------------------