PR Dashboard: Live PR status comment

[trask]

Problem

The dashboard knows who has the next action on a PR, but that status is hard to find from the PR conversation itself.

Proposal

Maintain one dashboard-managed status comment on each PR after the repository has dashboard state.

The comment should be created on targeted PR updates, including draft PRs, and updated in place on later targeted updates.

This should replace the existing one-time guidance comment.

It should show:

• who has the next action
• what the PR is waiting on
• links to unresolved author-action discussions when the PR is waiting on the author

It should also include brief author guidance to make sure every review discussion has a clear outcome.

Onboarding behavior

Ignore targeted PR updates until the repository's first full dashboard run has populated dashboard state.

Acceptance criteria

• Each PR has at most one dashboard-managed status comment.
• The comment is created or updated only after dashboard state exists.
• Draft PRs are supported.
• Waiting-on-author comments link to relevant unresolved discussions when possible.
• Pre-bootstrap targeted PR updates are ignored.