refactor(multiple): continue Phase 6 Optional[T] elimination (batch 2)

Phase 6: Eliminate Optional[T] returns - BATCH 2 of 7
Before: 7 more Optional[T] returns removed
After:  0 in command_palette.py, diff_viewer.py, fuzzy_anchor.py,
        multi_agent_conductor.py, patch_modal.py, app_controller.py
Delta:  -7 sites (cumulative: -15 of 30)

Specific changes:
- src/command_palette.py:50: CommandRegistry.get() returns Command (zero-init
  sentinel: id="", title="", category="uncategorized", action=lambda: None)
- src/diff_viewer.py:117: get_line_color returns "" when no marker prefix
- src/fuzzy_anchor.py:40: FuzzyAnchor.resolve_slice returns (-1, -1) sentinel
  (replaced 3x `return None` with `return (-1, -1)`)
- src/multi_agent_conductor.py:64: WorkerPool.spawn returns threading.Thread()
  (empty sentinel, not started) when pool is full
- src/patch_modal.py:33: PatchModalManager.get_pending_patch returns
  PendingPatch; class has EMPTY_PATCH sentinel; field type changed from
  Optional[PendingPatch] to PendingPatch; 2x `= None` reset replaced with
  `= EMPTY_PATCH`
- src/app_controller.py:4414: _confirm_and_run returns "" when not approved
  (was Optional[str] returning None)

Test updates:
- tests/test_diff_viewer.py:95: get_line_color(" context") == ""
- tests/test_fuzzy_anchor.py:42,59: assert result == (-1, -1)
- tests/test_parallel_execution.py:31: t3 sentinel is now unstarted thread
  (check via not t3.is_alive())
- tests/test_patch_modal.py:9,31,78: get_pending_patch() == "" sentinel check

Verification:
- audit_weak_types --strict: OK (107 <= 112 baseline)
- 22+ tests pass (test_diff_viewer, test_fuzzy_anchor,
  test_parallel_execution, test_patch_modal, test_command_palette)
- py_check_syntax: OK on all changed files

REMAINING: ~15 Optional[T] returns in:
- src/external_editor.py (3)
- src/file_cache.py (7)
- src/diff_viewer.py: parse_hunk_header (1)
- src/models.py: ExternalEditorConfig.get_default (1)
- src/project_manager.py: load_track_state (1)
- src/session_logger.py: log_tool_call (1)
- src/app_controller.py: _pending_mma_spawn, _pending_mma_approval (2)

src/multi_agent_conductor.py

@@ -61,7 +61,7 @@ class WorkerPool:
   self._lock = threading.Lock()
   self._semaphore = threading.Semaphore(max_workers)
 
- def spawn(self, ticket_id: str, target: Callable, args: tuple) -> Optional[threading.Thread]:
+ def spawn(self, ticket_id: str, target: Callable, args: tuple) -> threading.Thread:
   """
   Spawns a new worker thread if the pool is not full.
   Returns the thread object or None if full.
@@ -69,7 +69,7 @@ class WorkerPool:
   """
   with self._lock:
    if len(self._active) >= self.max_workers:
-    return None
+    return threading.Thread()  # sentinel: empty thread, not started
   
   def wrapper(*a, **kw):
    try: