refactor(project): create src/project.py with ProjectContext + 5 sub + config IO (split from models.py)

Per the 4-criteria decision rule (C1=cross-system, C3=tests, C4=size);
ProjectContext is the typed return of project_manager.flat_config();
the 5 sub-dataclasses model the actual nested dict structure of
flat_config()'s return; load_config_from_disk / save_config_to_disk
are the canonical config I/O primitives (renamed from the private
_load_config_from_disk / _save_config_to_disk).

This commit:
 1. Creates src/project.py with ProjectContext + 5 sub (ProjectMeta,
    ProjectOutput, ProjectFiles, ProjectScreenshots, ProjectDiscussion)
    + EMPTY_PROJECT_CONTEXT + _clean_nones + load_config_from_disk +
    save_config_to_disk + parse_history_entries.
 2. Removes the original class + function definitions from src/models.py.
 3. Adds backward-compat re-exports in src/models.py (the same pattern
    used by Phase 3a mma.py and Phase 3g personas.py).
 4. Updates src/app_controller.py to use the new public function names
    (load_config_from_disk / save_config_to_disk).
 5. Updates tests/test_models_no_top_level_tomli_w.py to use the new
    public name (the test still asserts lazy-loading; the lazy load
    happens in the new project.py module).
 6. Updates scripts/audit_no_models_config_io.py FORBIDDEN_PATTERNS to
    reference the new public names (models.load_config_from_disk /
    models.save_config_to_disk) + the new src.project path.

Verification: VC6
  uv run python -c 'from src.project import ProjectContext, ProjectMeta,
  ProjectOutput, ProjectFiles, ProjectScreenshots, ProjectDiscussion,
  _clean_nones, load_config_from_disk, save_config_to_disk,
  parse_history_entries'  # OK
  uv run python -c 'from src.models import ProjectContext, ...'  # OK
  (re-exports work)

Pre-existing test regression (NOT caused by this commit):
  tests/test_models_no_top_level_tomli_w.py::test_models_does_not_import_tomli_w_at_module_level
  was already failing because the Phase 3g 'from src.personas import Persona'
  re-export in src/models.py loads src.personas at module level, which
  loads tomli_w. The Phase 5 reduce-models.py pass moves the persona
  import into __getattr__ (lazy), which will make this test pass again.

Tests verified: tests/test_project_context_20260627.py (10/10 PASS),
tests/test_project_serialization.py (2/2 PASS), tests/test_thinking_persistence.py
(4/4 PASS), tests/test_presets.py (3/3 PASS), tests/test_persona_models.py
(2/2 PASS), tests/test_ticket_queue.py (PASS), tests/test_dag_engine.py
(PASS), tests/test_orchestration_logic.py (PASS).

tests/test_models_no_top_level_tomli_w.py

@@ -46,7 +46,7 @@ def test_models_can_still_call_save_config_after_lazy_load() -> None:
   "theme": {"palette": "solarized_dark", "font_size": 16.0},
  }
  try:
-  src.models._save_config_to_disk(config)
+  src.models.save_config_to_disk(config)
  except Exception as e:
   pytest.fail(f"save_config raised after lazy tomli_w: {e}")
  finally:
@@ -63,7 +63,7 @@ def test_save_config_uses_tomli_w_on_demand() -> None:
  assert "tomli_w" not in sys.modules
  # Call save_config - this should trigger the import
  try:
-  src.models._save_config_to_disk({"test_key": "test_value"})
+  src.models.save_config_to_disk({"test_key": "test_value"})
  except Exception:
   # We don't care if the save itself fails; we just want to verify
   # the import happened.