refactor(models,paths,presets,summary_cache): remove Optional returns (Phase 6 batch 1)

Phase 6: Eliminate Optional[T] returns (FR5) - BATCH 1 of 7
Before: 8 Optional[T] return types across 4 files
After:  0 (replaced with default-zero return values)
Delta:  -8 sites

Per conductor/code_styleguides/error_handling.md "Optional[X] ban":
- "Use Result[T] for any function that can fail at runtime."
- "Use nil-sentinel dataclasses for 'no result'."

For accessor-style returns (lookup or zero-default), convert to:
- Optional[str] -> str with default "" (empty string sentinel)
- Optional[float] -> float with default 0.0
- Optional[int] -> int with default 0
- Optional[Path] -> Path with default Path("") or project_root

Specific changes:
- src/models.py:765-789: Persona.provider/model/temperature/top_p/max_output_tokens
  (Optional[str]/[float]/[int] -> str/float/int with default zero values)
- src/paths.py:255: _get_project_conductor_dir_from_toml returns project_root
  when no [conductor].dir override is configured (was Optional[Path] returning None)
- src/presets.py:21: project_path property returns Path("") when no project_root
  (was Optional[Path] returning None)
- src/summary_cache.py:57: get_summary returns "" when hash mismatch (was
  Optional[str] returning None)

Test updates:
- tests/test_persona_models.py:64-69: test_persona_defaults now expects
  "" / 0.0 instead of None
- tests/test_summary_cache.py:25, 32, 58: get_summary assertions now
  expect "" instead of None

Verification:
- audit_weak_types --strict: OK (107 <= 112 baseline)
- 13 tests pass (test_summary_cache, test_paths, test_presets,
  test_persona_models)
- py_check_syntax: OK on all changed files

REMAINING: ~22 Optional[T] returns in:
- src/command_palette.py (1)
- src/diff_viewer.py (2)
- src/external_editor.py (3)
- src/file_cache.py (7)
- src/fuzzy_anchor.py (1)
- src/models.py (1)
- src/multi_agent_conductor.py (1)
- src/patch_modal.py (1)
- src/project_manager.py (1)
- src/session_logger.py (1)
- src/app_controller.py (3)

src/summary_cache.py

@@ -54,9 +54,9 @@ class SummaryCache:
   except OSError as e:
    return Result(data=False, errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e), source="summary_cache.save", original=e)])
 
- def get_summary(self, file_path: str, content_hash: str) -> Optional[str]:
+ def get_summary(self, file_path: str, content_hash: str) -> str:
   """
-  Returns cached summary if hash matches, otherwise None.
+  Returns cached summary if hash matches, otherwise "".
   [C: tests/test_summary_cache.py:test_summary_cache, tests/test_summary_cache.py:test_summary_cache_lru]
   """
   entry = self.cache.get(file_path)
@@ -64,8 +64,8 @@ class SummaryCache:
    # LRU: move to end
    val = self.cache.pop(file_path)
    self.cache[file_path] = val
-   return val.get("summary")
-  return None
+   return val.get("summary") or ""
+  return ""
 
  def set_summary(self, file_path: str, content_hash: str, summary: str) -> None:
   """