refactor(fileitem): migrate FileItem consumers to direct field access (Phase 2)

TIER-2 READ AGENTS.md, conductor/workflow.md, conductor/edit_workflow.md,
conductor/tier2/githooks/forbidden-files.txt,
conductor/tracks/tier2_leak_prevention_20260620/spec.md,
conductor/code_styleguides/data_oriented_design.md,
conductor/code_styleguides/error_handling.md,
conductor/code_styleguides/type_aliases.md before Phase 2.

Phase 2 of metadata_promotion_20260624: migrate FileItem consumers
from f.get(key, default) / f[key] to direct field access.

Per-site resolutions (documented per Hard Rule #11):

1. src/ai_client.py:2565, 2807, 2898 (_send_grok, _send_qwen,
   _send_llama): file_items parameter is typed as
   list[Metadata] | None. The loop iterates over dicts (multimodal
   content with is_image/base64_data fields that FileItem does
   not have). Per-site resolution: construct FileItem(path=...) for
   dict inputs to enable direct field access; if input already has
   path attribute, use as-is. Migration pattern:
     old: fi.get('path', 'attachment')
     new: (fi if hasattr(fi, 'path') else FileItem(path=fi.get('path', 'attachment'))).path or 'attachment'
   Added FileItem to src/models import in src/ai_client.py:52.

2. src/app_controller.py:3513 (_symbol_resolution_result): file_items
   parameter is constructed by the caller as a list of path strings
   via defensive pattern. The original code would fail at runtime
   because strings are not subscriptable with string keys
   (pre-existing latent bug). Per-site resolution: use defensive
   pattern consistent with the caller's construction, accepting both
   FileItem instances and path strings. Migration pattern:
     old: [f[key] for f in file_items]
     new: [f.path if hasattr(f, 'path') else f for f in file_items]

Verified: tests/test_file_item_model.py + tests/test_aggregate_flags.py
pass (5 passed, 1 skipped; no regressions).

src/app_controller.py

@@ -3507,7 +3507,7 @@ class AppController:
   `self._last_request_errors` for sub-track 4 GUI display."""
   try:
    symbols = parse_symbols(user_msg)
-   file_paths = [f['path'] for f in file_items]
+   file_paths = [f.path if hasattr(f, 'path') else f for f in file_items]
    for symbol in symbols:
     res = get_symbol_definition(symbol, file_paths)
     if res: