refactor(src): Phase 10.2 batch 1 - session_logger + file_cache Result[T] migration

Migrates 5 SILENT_SWALLOW sites to full Result[T] pattern:

session_logger.py (4 sites):
1. log_api_hook - returns Result[bool] (was None)
2. log_comms - returns Result[bool] (was None)
3. log_tool_call - returns Result[Optional[str]] (was Optional[str])
4. log_cli_call - returns Result[bool] (was None)

file_cache.py (1 site):
- L98: removed dead code (try/except StopIteration around
  next(iter(_ast_cache)) is unreachable because we just checked
  len(_ast_cache) >= 10)

Updates tests/test_session_logger_optimization.py to extract
result.data from the new Result-based API.

All callers of these log_* functions previously ignored the
return value; they continue to ignore the new Result return
value (backwards-compatible).

tests/test_session_logger_optimization.py

@@ -51,19 +51,21 @@ def test_log_tool_call_saves_in_session_scripts(temp_session_setup: tuple[Path,
  script_content = "Write-Host 'Hello from test'"
  result_content = "Success"
  
- # Call log_tool_call with script_path=None
- ps1_path_str = session_logger.log_tool_call(script_content, result_content, None)
- assert ps1_path_str is not None
- 
- ps1_path = Path(ps1_path_str)
+ # Call log_tool_call with script_path=None. Returns Result[Optional[str]].
+ ps1_result = session_logger.log_tool_call(script_content, result_content, None)
+ assert ps1_result.ok, f"log_tool_call failed: {ps1_result.errors}"
+ assert ps1_result.data is not None
+
+ ps1_path = Path(ps1_result.data)
  assert ps1_path.parent == scripts_subdir
  assert ps1_path.name == "script_0001.ps1"
  assert ps1_path.read_text(encoding="utf-8") == script_content
- 
+
  # Verify second call increments sequence
- ps1_path_str_2 = session_logger.log_tool_call("Get-Date", "2026-03-08", None)
- assert ps1_path_str_2 is not None
- assert Path(ps1_path_str_2).name == "script_0002.ps1"
+ ps1_result_2 = session_logger.log_tool_call("Get-Date", "2026-03-08", None)
+ assert ps1_result_2.ok, f"log_tool_call failed: {ps1_result_2.errors}"
+ assert ps1_result_2.data is not None
+ assert Path(ps1_result_2.data).name == "script_0002.ps1"
 
 def test_log_tool_output_saves_in_session_outputs(temp_session_setup: tuple[Path, Path]) -> None:
  log_dir, _ = temp_session_setup