feat(ingestion): implement brain_ingest_raw MCP tool

Wraps pipeline.RunRaw directly. Same dry-run semantics as the HTTP
/ingest-raw endpoint. Test exercises a single concept page; asserts
returned path and that no file is written under dry_run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/server.go

@@ -120,6 +120,8 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainQuery(ctx, args)
 	case "brain_write":
 		return s.brainWrite(ctx, args)
+	case "brain_ingest_raw":
+		return s.brainIngestRaw(ctx, args)
 	default:
 		return nil, fmt.Errorf("unknown tool: %s", name)
 	}