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/fuzzy_anchor.py

@@ -37,7 +37,9 @@ class FuzzyAnchor:
   }
 
  @classmethod
- def resolve_slice(cls, text: str, slice_data: dict) -> Optional[Tuple[int, int]]:
+ def resolve_slice(cls, text: str, slice_data: dict) -> Tuple[int, int]:
+  """Returns (start_line, end_line) on success, or (-1, -1) if unresolved."""
+  result: Tuple[int, int] = (-1, -1)
   """
   [C: tests/test_fuzzy_anchor.py:TestFuzzyAnchor.test_resolve_slice_anchor_mismatch_returns_none, tests/test_fuzzy_anchor.py:TestFuzzyAnchor.test_resolve_slice_exact_match, tests/test_fuzzy_anchor.py:TestFuzzyAnchor.test_resolve_slice_line_deleted_before_returns_none, tests/test_fuzzy_anchor.py:TestFuzzyAnchor.test_resolve_slice_line_inserted_before, tests/test_fuzzy_anchor.py:TestFuzzyAnchor.test_resolve_slice_multiple_lines_changed]
   """
@@ -54,7 +56,7 @@ class FuzzyAnchor:
   # 2. Fuzzy match
   start_ctx = slice_data["start_context"]
   end_ctx   = slice_data["end_context"]
-  if not start_ctx or not end_ctx: return None
+  if not start_ctx or not end_ctx: return (-1, -1)
 
   # Search for start_ctx
   best_s = -1
@@ -68,7 +70,7 @@ class FuzzyAnchor:
     best_s = i
     break
   
-  if best_s == -1: return None
+  if best_s == -1: return (-1, -1)
 
   # Search for end_ctx after start_ctx
   best_e = -1
@@ -87,4 +89,4 @@ class FuzzyAnchor:
   if best_e != -1:
    return (best_s + 1, best_e)
   
-  return None
+  return (-1, -1)