rebind the Claude pane on an in-place workspace switch (T-269)

Separate clide windows are isolated (own process, per-root IPC socket,
per-repo deterministic session id), so parallel repos in separate windows
were already fine. But switching the workspace in place (Open Project /
Open Folder) only emitted ProjectOpened — nothing rebound the Claude
session, so the primary pane kept the PREVIOUS repo's conversation.

Two compounding causes, fixed in layers:
- ClaudeSessionOrchestrator.spawn() was idempotent on the literal key
  'primary' without checking cwd, so it handed the old repo's session to
  the new repo. It now reuses a cached session only when its cwd matches
  the spec; a mismatch tears the stale one down and spawns fresh.
- The primary ClaudePane is built once behind a GlobalKey and spawns once,
  so it never re-resolved. It now listens for ProjectOpened and rebinds:
  close its orchestrator entry, drop the cached session id + repo root, and
  respawn against the now-active repo. Secondaries don't self-rebind.
- ClaudeSessionHost drops the old repo's secondary/fork tabs on a switch,
  so a switched workspace starts like a fresh launch (lone primary).
- The extension closes any remaining sessions whose cwd != the new root,
  catching team/non-pane sessions no pane owns.

Tested at the orchestrator: cwd-aware idempotency (reuse on cwd match,
teardown + respawn on mismatch). Pane/host widget coverage is intentionally
deferred — claude_pane.dart has no widget-test harness yet and pulling it
into coverage piecemeal would drop the gate; tracked separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/claude/session_lifecycle_test.dart

@@ -47,11 +47,11 @@ ClaudeSessionOrchestrator _orch(List<_FakeProc> created) {
   );
 }
 
-SpawnSpec _spec(String id, {bool resume = false, String? transcriptPath}) => SpawnSpec(
+SpawnSpec _spec(String id, {bool resume = false, String? transcriptPath, String cwd = '/repo'}) => SpawnSpec(
       id: id,
       role: id,
       sessionId: '$id-uuid',
-      cwd: '/repo',
+      cwd: cwd,
       resume: resume,
       transcriptPath: transcriptPath,
     );
@@ -140,6 +140,40 @@ void main() {
     });
   });
 
+  // ---- in-place workspace switch (T-269) ---------------------------------
+
+  group('spawn — cwd-aware idempotency (T-269)', () {
+    late List<_FakeProc> created;
+    late ClaudeSessionOrchestrator orch;
+
+    setUp(() {
+      created = [];
+      orch = _orch(created);
+    });
+
+    tearDown(() => orch.dispose());
+
+    test('same id + same cwd reuses the session (no second process)', () async {
+      final a = await orch.spawn(_spec('primary', cwd: '/repo-a'));
+      final b = await orch.spawn(_spec('primary', cwd: '/repo-a'));
+      expect(identical(a, b), isTrue);
+      expect(created, hasLength(1));
+    });
+
+    test('same id + different cwd tears down the old session and spawns fresh', () async {
+      // Workspace switched in place: the new repo must NOT inherit the old
+      // repo's 'primary' session.
+      final a = await orch.spawn(_spec('primary', cwd: '/repo-a'));
+      final b = await orch.spawn(_spec('primary', cwd: '/repo-b'));
+      expect(identical(a, b), isFalse);
+      expect(b.cwd, '/repo-b');
+      expect(created, hasLength(2));
+      expect(created.first.killed, isTrue, reason: 'old repo session is killed');
+      expect(orch.sessions, hasLength(1));
+      expect(orch.byId('primary')!.cwd, '/repo-b');
+    });
+  });
+
   // ---- /resume — bind to an existing session via orchestrator -------------
 
   group('/resume — resume an existing session via orchestrator', () {