refactor(rag_engine,ai_client): rag_engine.search returns List[RAGChunk] directly

Phase 5: rag_engine.search() return type (FR4 row 7)
Before: def search(...) -> List[Dict[str, Any]] at src/rag_engine.py:367
After:  def search(...) -> List["RAGChunk"]
Delta:  -1 wrong type annotation (List[Dict] -> List[RAGChunk])

RAGChunk dataclass extended with `id: str = ""` field to preserve the
chroma wire-format identifier. The search() function now constructs
RAGChunk instances directly from chromadb query results, normalizing
the wire format (metadata.path -> RAGChunk.path; distance -> 1.0 - score)
at the boundary.

Consumer updates:
- src/ai_client.py:3259-3266: chunk["metadata"]["path"] -> chunk.path;
  chunk["document"] -> chunk.document (direct attribute access)
- src/app_controller.py:3506: docstring updated from Result[List[Dict]]
  to Result[List[RAGChunk]] (no code change; pass-through)

Test updates:
- tests/test_rag_engine.py:61: results[0]["id"] -> results[0].id
  (now uses dataclass attribute access)

Verification:
- audit_weak_types --strict: OK (107 <= 112 baseline)
- py_check_syntax: OK on rag_engine.py, ai_client.py, test_rag_engine.py
- 21 RAG tests pass (test_rag_engine, test_rag_chunk,
  test_rag_engine_ready_status_bug, test_rag_integration,
  test_context_composition_decoupled, test_tiered_aggregation)

src/ai_client.py

@@ -3260,9 +3260,8 @@ def send(
   if chunks:
    context_block = "## Retrieved Context\n\n"
    for i, chunk in enumerate(chunks):
-    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 ""
+    path = chunk.path if chunk.path else "unknown"
+    doc = chunk.document
     context_block += f"### Chunk {i+1} (Source: {path})\n{doc}\n\n"
    user_message = context_block + user_message