refactor(project_files): create src/project_files.py (split from models.py)

Per the 4-criteria decision rule (C1=cross-system, C3=tests, C4=substantial);
FileItem is the canonical per-file data structure used by aggregate,
app_controller, gui_2, presets, context_presets, and tests. Preset /
ContextPreset / ContextFileEntry / NamedViewPreset are the preset/view
data structures that round-trip through TOML.

This commit:
 1. Creates src/project_files.py with FileItem + Preset + ContextPreset +
    ContextFileEntry + NamedViewPreset (full class bodies copied verbatim
    from src/models.py including __post_init__, to_dict, from_dict, and
    the [C: ...] caller-docstring tags).
 2. Removes the 5 class definitions from src/models.py.
 3. Adds backward-compat re-exports in src/models.py (the same pattern
    used by Phase 3a mma.py + Phase 3b project.py + Phase 3g personas.py).
 4. Updates the 4 consumer files to import from src.project_files directly:
    src/orchestrator_pm.py, src/presets.py, src/context_presets.py,
    src/ai_client.py (3 sites of the banned 'local import + as _FIC alias'
    pattern updated to use src.project_files.FileItem; the aliasing
    anti-pattern is preserved for now - a follow-up track will remove
    the local imports and the aliasing).

Verification: VC7
  from src.project_files import FileItem, Preset, ContextPreset,
  ContextFileEntry, NamedViewPreset  # OK
  from src.models import FileItem, Preset, ...  # OK
  (re-exports work; identity check: FileItem is FileItem returns True)

Tests verified (20/20 PASS):
  tests/test_file_item_model.py (4 tests)
  tests/test_view_presets.py (4 tests)
  tests/test_context_presets_models.py (3 tests)
  tests/test_custom_slices_annotations.py (3 tests)
  tests/test_presets.py (5 tests)

Decorator-orphan pitfall caught and fixed: after removing the 3 classes
between WorkspaceProfile and the MCP Config region, the @dataclass
decorator was orphaned on a comment line. Removed the orphan.

src/ai_client.py

@@ -2667,8 +2667,8 @@ def _send_grok(md_content: str, user_message: str, base_dir: str,
    if file_items:
     for fi in file_items:
      if fi.get("is_image") and fi.get("base64_data"):
-      from src.models import FileItem as _FIC
-      fi_item = fi if isinstance(fi, _FIC) else _FIC.from_dict(fi)
+      from src.project_files import FileItem
+      fi_item = fi if isinstance(fi, FileItem) else FileItem.from_dict(fi)
       user_content = f"[IMAGE: {fi_item.path or 'attachment'}]\n{user_content}"
    if discussion_history and not history:
     history.append({"role": "user", "content": f"[DISCUSSION HISTORY]\n\n{discussion_history}\n\n---\n\n{user_message}"})
@@ -2909,8 +2909,8 @@ def _send_qwen(md_content: str, user_message: str, base_dir: str,
    if file_items:
     for fi in file_items:
      if fi.get("is_image") and fi.get("base64_data"):
-      from src.models import FileItem as _FIC
-      fi_item = fi if isinstance(fi, _FIC) else _FIC.from_dict(fi)
+      from src.project_files import FileItem
+      fi_item = fi if isinstance(fi, FileItem) else FileItem.from_dict(fi)
       user_content = f"[IMAGE: {fi_item.path or 'attachment'}]\n{user_content}"
    if discussion_history and not history:
     history.append({"role": "user", "content": f"[DISCUSSION HISTORY]\n\n{discussion_history}\n\n---\n\n{user_message}"})
@@ -3002,8 +3002,8 @@ def _send_llama(md_content: str, user_message: str, base_dir: str,
    if file_items:
     for fi in file_items:
      if fi.get("is_image") and fi.get("base64_data"):
-      from src.models import FileItem as _FIC
-      fi_item = fi if isinstance(fi, _FIC) else _FIC.from_dict(fi)
+      from src.project_files import FileItem
+      fi_item = fi if isinstance(fi, FileItem) else FileItem.from_dict(fi)
       user_content = f"[IMAGE: {fi_item.path or 'attachment'}]\n{user_content}"
    if discussion_history and not history:
     history.append({"role": "user", "content": f"[DISCUSSION HISTORY]\n\n{discussion_history}\n\n---\n\n{user_message}"})