PR Dashboard: Let authors route a PR to reviewers

[trask]

Problem

Automatic routing can leave a pull request in Waiting on authors (or Waiting on an external dependency) when the author actually needs attention from reviewers. Authors often cannot apply labels themselves to redirect it.

Proposal

Support a reviewer-routing override command on pull requests:

• /dashboard route:reviewers moves the pull request to Waiting on reviewers.
• The command is accepted from the pull request author or a member of a configured approver team (approver_teams).
• It takes effect only while the pull request's automatic route is Waiting on authors or Waiting on an external dependency (the pre-review routes). On any route already at or past reviewers it is a no-op.
• While effective it applies the dashboard:route-overridden label and the pull request is shown in Waiting on reviewers.
• The override is released automatically once automatic routing reaches or passes reviewers, so a forgotten label cannot pin the pull request at reviewers or drag it back from maintainers. Removing the label manually also returns control to automatic routing.
• The override is applied before the Copilot review gate, so it never bypasses a required Copilot review.
• The command line is treated as control metadata; any trailing explanation the author includes is preserved as normal activity.

Command replies

The dashboard posts a single reply (at most once per command) when:

• an unauthorized user issues the command,
• an unrecognized /dashboard subcommand is used, or
• an authorized command has no effect because the pull request is already at/past reviewers (an already routed reply).

Discoverability

The /dashboard route:reviewers command is advertised both in the author nudge and on the live PR status comment.

• Maintainer routing (/maintainer / Waiting on maintainers).
• Closed pull request handling (dashboard-closed PRs, reopening, keeping closed PRs visible).
• Stale labeling and automatic closure.