restore tmux session in Claude pane

Reverts the bare-shell diagnostic. Spawns tmux new-session -A -s
<sessionName> for persistence. Claude command will be added back
after tmux + resize are stable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-01T10:11:15Z",
+  "exported_at": "2026-05-01T10:20:03Z",
   "decisions": [
     {
       "id": "D-1",

lib/builtin/claude/src/claude_pane.dart

@@ -123,10 +123,14 @@ class _ClaudePaneState extends State<ClaudePane> {
         ? primarySessionName(repoRoot)
         : secondarySessionName(repoRoot, widget.secondaryIndex!);
 
-    // TODO: restore tmux+claude once resize is stable.
-    // Bare shell for resize debugging.
-    final shell = Platform.environment['SHELL'] ?? '/bin/zsh';
-    var argv = <String>[shell, '-l'];
+    // tmux-wrapped session for persistence (D-041).
+    var argv = <String>[
+      'tmux',
+      'new-session',
+      '-A',
+      '-s',
+      sessionName,
+    ];
     var resp = await ipc.request('pane.spawn', args: {
       'argv': argv,
       'kind': PaneKind.claude.wire,