Cleanup residual dead code and stale annotations (N-scan residual)

- Drop the "pass-through remains for back-compat" note on
  AppDatabaseBase's domain modules; the pass-through layer is gone, so
  the comment was out of date. Replace with a pointer to the ESLint
  guard that blocks regression.

- Flag the two remaining >500-line files (`agent-runtime/runtime-bridge.ts`
  648 LOC, `agent-runtime/artifact-projector.ts` 637 LOC) with an
  inline `TODO: decompose — M-followup` note. No structural change;
  the decomposition is tracked for a later milestone to keep this pass
  scope-safe.

src/main/agent-runtime/runtime-bridge.ts

@@ -1,3 +1,7 @@
+// TODO: decompose — M-followup. File is 648 lines; split into
+// sqlite-session-store wiring, cost/metrics routing, and hook-dispatch
+// assembly. Tracked as a future milestone; no structural change in this
+// pass.
 import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import type { AppDatabase } from "@main/database";
 import type { AgentMessageRecord, DesktopEvent } from "@shared/types";