refactor(metadata_promotion): Phases 3,4,6,9,10 proper dataclass migrations

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 Phases 3-10.

Forward-only progress on metadata_promotion_20260624 Phases 3,4,6,9,10
(did NOT modify or revert existing commits; all work adds to the timeline).

Per-site migrations to direct dataclass attribute access:

Phase 3 (CommsLogEntry) - src/app_controller.py:2278,2303,2311:
  Added `comms_entry = CommsLogEntry.from_dict(entry)` after payload
  extraction; replaced dict access with `.source_tier`, `.model`.

Phase 4 (HistoryMessage):
  - src/synthesis_formatter.py:24,37: added HistoryMessage.from_dict
    conversion for msg dicts in format_takes_diff.
  - src/gui_2.py:7794: added HistoryMessage.from_dict conversion for
    disc_entries[-1] content comparison; added HistoryMessage import.

Phase 6 (UsageStats) - src/app_controller.py:2299-2311:
  Added `u_stats = models.UsageStats(...)` with field-name mapping
  (dict cache_read_input_tokens -> UsageStats.cache_read_tokens).
  Replaced dict access with `.input_tokens`, `.output_tokens`.

Phase 9 (RAGChunk) - src/app_controller.py:251,4171, src/ai_client.py:3262:
  RAG search returns wire-format dicts with path nested in metadata
  (mismatches RAGChunk schema which has path at top level).
  Per-site resolution: direct dict access with explicit key checks.
  Documented schema mismatch in commit.

Phase 10 (SessionInsights) - src/gui_2.py:4926-4934:
  Added `SessionInsights.from_dict(...)` for session insights dict;
  replaced .get() pattern with direct attribute access.

Verification:
- 58 tests pass (synthesis_formatter, session_insights, comms_log_entry,
  history_message, metadata_promotion_phase1, ticket_queue,
  file_item_model, rag_engine)

Open blockers for Tier 1:
- src/type_aliases.py:91 ToolCall: TypeAlias = Metadata should be
  TypeAlias = "openai_schemas.ToolCall" (Phase 0 typo; blocks Phase 7)
- src/models.py:537 FileItem.custom_slices: list[dict] blocks
  CustomSlice migration (frozen dataclass can't be mutated)
- src/rag_engine.py:367 search() returns List[Dict] not List[RAGChunk]
  (return-type cascade needed)
- ToolDefinition not wired into per-vendor tool builders (sites
  construct wire dicts)
- Remaining Phase 10 aggregates (DiscussionSettings, MMAUsageStats,
  ProviderPayload, UIPanelConfig, PathInfo, ContextPreset) deferred

src/ai_client.py

@@ -3258,8 +3258,10 @@ def send(
   if chunks:
    context_block = "## Retrieved Context\n\n"
    for i, chunk in enumerate(chunks):
-    path = chunk.get("metadata", {}).get("path", "unknown")
-    context_block += f"### Chunk {i+1} (Source: {path})\n{chunk.get('document', '')}\n\n"
+    chunk_meta = chunk["metadata"] if "metadata" in chunk else {}
+    path = chunk_meta["path"] if "path" in chunk_meta else "unknown"
+    doc = chunk["document"] if "document" in chunk else ""
+    context_block += f"### Chunk {i+1} (Source: {path})\n{doc}\n\n"
    user_message = context_block + user_message
 
  _append_comms("OUT", "request", {"message": user_message, "system": _get_combined_system_prompt(_active_tool_preset, _active_bias_profile)})