Host agents removed from the UI would reappear on the next report cycle
because there was no rejection mechanism — unlike Docker agents which
already had resurrection prevention. Mirror the Docker agent pattern:
- Track removed host IDs in a `removedHosts` map with 24hr TTL
- Persist removal records in `State.RemovedHosts` for frontend display
- Reject reports from removed hosts in `ApplyHostReport()`
- Add `AllowHostReenroll()` + API route to clear the block
- Show removed host agents in the Settings UI with "Allow re-enroll"
- Sync removed-agent maps from state on startup for all agent types
- Fix mock integration snapshot missing `RemovedDockerHosts` field