fix: pre-push review — operator-context retry walk, get_content offload, doc cleanups

- A1: add a shared `operator-context` marker to every operator-context row in
  both UIs; the retry-skip walk keys on it, so a trailing watch-result /
  guard-finding / idle-children card no longer makes retry regenerate the
  wrong turn. Pinned by source-grep tests + a headless-DOM self-test.
- A2: wrap get_content's two sync DB gates (get_attachment + the unbounded
  ws-scoped attachment_referenced_in_ws LIKE scan) in asyncio.to_thread so a
  long scan can't stall the event loop — matching the module's convention.
- A3: add the HistoryEvent attachments docstring bullet; drop the dead
  `interjection` class; document the interactive-only system-context label;
  correct the SDK attachments-meta docs to {kind, filename, mime_type}
  (size_bytes is not carried through the history projection).

sdk/typescript/src/events.ts

@@ -26,7 +26,7 @@ export interface HistoryEvent {
    *   (e.g. `watch_triggered`'s `{watch_name, command, poll_count, max_polls,
    *   is_final}`) so the renderer can rebuild per-kind UI (the watch-result
    *   card); absent for kinds with no structured data
-   * - `attachments`: per-attachment metadata `{kind, filename, mime_type, size_bytes}`
+   * - `attachments`: per-attachment metadata `{kind, filename, mime_type}`
    * - `reasoning`: concatenated reasoning text for assistant turns that
    *   round-tripped a thinking-block lane (Anthropic-with-thinking today;
    *   OpenAI Responses + Gemini in later phases). Present only when the