Give Guardian reviewers read-only access to parent history (#40848)

## Why

Guardian approval reviews may need the original user instructions to decide
whether a command is authorized.

## What changed

- Forward the parent thread's `history.list_windows`, `history.list_items`,
  `history.read_item`, and `history.search_contents` executors to Guardian
  reviewer sessions.
- Keep other parent extension tools, including the `notes` namespace, isolated
  from Guardian reviewers.

## Testing

Add an app-server integration test that reads parent history during a Guardian
review and verifies that `notes` tools are not exposed.

GitOrigin-RevId: 65a90036577050adfa23b1d98f862233c994d9d7