Add MCP server, note mutation endpoint, and updated_at tracking (v3.0.0)

New MCP server (mcp/) exposes kb operations as native MCP tools over
Streamable HTTP with Bearer token auth. Supports collections via tag
conventions, chunked file uploads, and agent-side search patterns.

Engine gains PATCH /api/v1/notes/{id} for in-place note updates with
transactional re-chunk/re-embed, and updated_at column on documents.

Go client adds updatenote command and Patch HTTP method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

engine/main.py

@@ -62,7 +62,7 @@ async def lifespan(app: FastAPI):
 app = FastAPI(title="kb-engine", version=__version__, lifespan=lifespan)
 
 # Import routes after app is created
-from kb.routes import health, search, jobs, documents, tags, status, reindex, auth  # noqa: E402, F401
+from kb.routes import health, search, jobs, documents, tags, status, reindex, auth, notes  # noqa: E402, F401
 
 if __name__ == "__main__":
     import uvicorn