PR watch: wake once when pending checks stop progressing?

[griffemi]

A watched PR whose checks stay pending never wakes the agent again. A stuck runner, a job waiting on approval, or a queued workflow that never starts leaves the thread waiting with no signal, and the agent has nothing to act on until a push, comment or conflict.

Proposal: wake the agent once when the PR's check state has not changed for 30 minutes. The wake lists the still-pending checks. It fires once per unchanged state; any check progressing or a new head resets it. It does not treat pending checks as failed or suggest merging.

Questions before implementing anything:

1. Is a stall reminder wanted at all?
2. Is 30 minutes right, or should it scale with how long checks usually take?
3. Should it be on by default, opt-in, or per watch?

Related bug fixes, which don't depend on this: #33450, #33451, #33452. No stall alert is implemented in any of them.