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)

tests/test_summary_cache.py

@@ -22,14 +22,14 @@ def test_summary_cache(tmp_path):
  summary = "**Python** - 1 lines"
  
  # Test empty cache
- assert cache.get_summary(file_path, content_hash) is None
+ assert cache.get_summary(file_path, content_hash) == ""
  
  # Test set and get
  cache.set_summary(file_path, content_hash, summary)
  assert cache.get_summary(file_path, content_hash) == summary
  
  # Test cache invalidation
- assert cache.get_summary(file_path, "different_hash") is None
+ assert cache.get_summary(file_path, "different_hash") == ""
  
  # Test persistence
  cache2 = SummaryCache(str(cache_file))
@@ -47,7 +47,7 @@ def test_summary_cache_lru(tmp_path):
  cache.set_summary("file2.py", "hash2", "summary2")
  cache.set_summary("file3.py", "hash3", "summary3") # This should evict file1.py
  
- assert cache.get_summary("file1.py", "hash1") is None
+ assert cache.get_summary("file1.py", "hash1") == ""
  assert cache.get_summary("file2.py", "hash2") == "summary2"
  assert cache.get_summary("file3.py", "hash3") == "summary3"
  
@@ -55,7 +55,7 @@ def test_summary_cache_lru(tmp_path):
  cache.get_summary("file2.py", "hash2")
  cache.set_summary("file4.py", "hash4", "summary4")
  
- assert cache.get_summary("file3.py", "hash3") is None
+ assert cache.get_summary("file3.py", "hash3") == ""
  assert cache.get_summary("file2.py", "hash2") == "summary2"
  assert cache.get_summary("file4.py", "hash4") == "summary4"