fix(app_controller): clear undo/redo history in btn_reset

test_undo_redo_lifecycle in tests/test_undo_redo_sim.py was failing in
the live_gui batch (passes in isolation) because btn_reset
(_handle_reset_session) was not clearing the HistoryManager's undo and
redo stacks. Prior tests in the same live_gui session leave stale
entries that interfere with tests that assume btn_reset provides a
clean history baseline.

Adds clearing of:
- app.history._undo_stack
- app.history._redo_stack
- app._last_ui_snapshot (None so the next take sets the baseline)
- app._pending_snapshot (False so debounce starts fresh)
- app._state_to_push (None so no stale state queued for push)

at the end of _handle_reset_session. The App is reached via
self.hook_server.app (set during _init_ai_and_hooks); all accesses are
guarded with hasattr/getattr for safety when the hook_server isn't
initialized yet (tests that construct AppController without starting
services).

Tests: test_undo_redo_lifecycle, test_undo_redo_discussion_mutation,
test_undo_redo_context_mutation all pass (3/3). The previously-failing
batch context also passes (verified with 14 tests from the gw7 worker
plus the test_undo_redo_sim set).

src/app_controller.py

@@ -3954,6 +3954,18 @@ class AppController:
   self.temperature = 0.0
   self.top_p = 1.0
   self.max_tokens = 8192
+  # Clear undo/redo history so the next session starts fresh. Without
+  # this, prior tests in the same live_gui session leave stale entries
+  # that interfere with tests like tests/test_undo_redo_sim.py that
+  # assume btn_reset provides a clean history baseline.
+  if hasattr(self, 'hook_server') and self.hook_server and getattr(self.hook_server, 'app', None) is not None:
+   app = self.hook_server.app
+   if hasattr(app, 'history'):
+    app.history._undo_stack.clear()
+    app.history._redo_stack.clear()
+   if hasattr(app, '_last_ui_snapshot'):   app._last_ui_snapshot = None
+   if hasattr(app, '_pending_snapshot'):   app._pending_snapshot = False
+   if hasattr(app, '_state_to_push'):      app._state_to_push  = None
 
  def _handle_compress_discussion(self) -> None:
   def worker() -> "Result[None]":