refactor(multiple): complete Phase 6 Optional[T] elimination (batches 4 + 5)

Phase 6: Eliminate Optional[T] returns - BATCHES 4 + 5 (FINAL)
Before: 11 more Optional[T] returns removed (Phase 6 total: 30 of 30)
After:  0 (Phase 6 COMPLETE per VC5)
Delta:  -11 sites in this commit; cumulative -30/30 sites across all batches

Specific changes:
- src/diff_viewer.py:27: parse_hunk_header returns (-1, -1, -1, -1) sentinel
  on parse failure (2x `return None` -> `return (-1, -1, -1, -1)`)
- src/external_editor.py:23,84,97: get_editor / _find_vscode_common_paths /
  auto_detect_vscode all return TextEditorConfig or str with zero-init
  defaults (no longer Optional)
- src/external_editor.py:48: launch_diff_result sentinel check changed from
  `if not editor:` to `if not editor.name or not editor.path:`
- src/file_cache.py:549,608,646,705,799,858: 6 nested walk/deep_search
  helper functions now return tree_sitter.Node (root) instead of
  Optional[tree_sitter.Node] (None)
- src/models.py:691,728: TextEditorConfig defaults added (name="", path="");
  EMPTY_TEXT_EDITOR_CONFIG sentinel; ExternalEditorConfig.get_default
  returns EMPTY_TEXT_EDITOR_CONFIG when no editors configured
- src/file_cache.py:895: get_file_id returns "" (was Optional[str])

Test updates:
- tests/test_diff_viewer.py: still passes (parse_hunk_header tested)
- tests/test_external_editor.py:78,97: is None -> == "" check (config.get_default,
  get_editor for unknown name)

Verification:
- audit_weak_types --strict: OK (107 <= 112 baseline)
- py_check_syntax: OK on all changed files
- 85+ tests pass (test_file_cache, test_ast_parser, test_external_editor,
  test_diff_viewer, test_fuzzy_anchor, test_summary_cache, test_paths,
  test_persona_models, test_patch_modal, test_parallel_execution,
  test_track_state_persistence, test_session_logger_optimization,
  + 117 in broader run)

VC5 (Zero Optional[T] return types) PASSES:
  git grep -cE "-> Optional\\[" -- 'src/*.py' returns 0

PHASE 6 IS COMPLETE.

REMAINING WORK:
- Phase 7: Eliminate Any + dict[str, Any] in internal signatures (59+ sites)
- Phase 8: Final re-measure + verification
- Phase 9: Boundary layer audit (done)

tests/test_external_editor.py

@@ -75,7 +75,7 @@ class TestExternalEditorConfig:
 
  def test_get_default_returns_none_when_empty(self):
   config = ExternalEditorConfig(editors={})
-  assert config.get_default() is None
+  assert config.get_default().name == ""
 
  def test_to_dict(self, ext_config):
   result = ext_config.to_dict()
@@ -94,7 +94,7 @@ class TestExternalEditorLauncher:
 
  def test_get_editor_unknown_name(self, launcher):
   editor = launcher.get_editor("unknown")
-  assert editor is None
+  assert editor.name == ""
 
  def test_build_diff_command(self, launcher, vscode_editor):
   cmd = launcher.build_diff_command(vscode_editor, "orig.txt", "mod.txt")