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/app_controller.py

@@ -3497,7 +3497,7 @@ class AppController:
 
  def _rag_search_result(self, user_msg: str) -> "Result[list[Metadata]]":
   """Per-event handler (Phase 6 Group 6.6): RAG search via the engine.
-  Returns Result[List[Dict]]. On failure: any engine/SDK exception
+  Returns Result[List[RAGChunk]]. On failure: any engine/SDK exception
   -> ErrorInfo(original=e). Caller (`_handle_request_event`) appends
   to `self._last_request_errors` for sub-track 4 GUI display."""
   if not (self.rag_engine and self.rag_config and self.rag_config.enabled):
@@ -4411,7 +4411,7 @@ class AppController:
   if self.ui_auto_scroll_tool_calls:
    self._scroll_tool_calls_to_bottom = True
 
- def _confirm_and_run(self, script: str, base_dir: str, qa_callback: Optional[Callable[[str], str]] = None, patch_callback: Optional[Callable[[str, str], Result[str]]] = None) -> Optional[str]:
+ def _confirm_and_run(self, script: str, base_dir: str, qa_callback: Optional[Callable[[str], str]] = None, patch_callback: Optional[Callable[[str, str], Result[str]]] = None) -> str:
   """
     [C: tests/test_arch_boundary_phase2.py:TestArchBoundaryPhase2.test_mutating_tool_triggers_callback, tests/test_arch_boundary_phase2.py:TestArchBoundaryPhase2.test_rejection_prevents_dispatch]
   """
@@ -4444,7 +4444,7 @@ class AppController:
      del self._pending_actions[dialog._uid]
   if not approved:
    self._append_tool_log(final_script, "REJECTED by user")
-   return None
+   return ""
   self.ai_status = "running powershell..."
   output = shell_runner.run_powershell(final_script, base_dir, qa_callback=qa_callback, patch_callback=patch_callback)
   self._append_tool_log(final_script, output)