feat: make PR follow-up a first-class task type

[joryirving]

Parent: #392

Make existing PR follow-up work a first-class Dispatch task before new issue pickup.

Signals:

• failing checks
• conflicts
• requested changes
• relevant new review or comment activity

Acceptance:

• next-task prefers follow-up over new work for the same agent
• follow-up task includes issue and PR context
• closed or completed PRs are skipped
• stale PRs are visible in Dispatch
• tests cover the main follow-up signals