Agent lifecycle: heartbeat, pause/checkpoint, and withdrawal

## Description

`src/agent/lifecycle.rs` contains three TODO items for core agent lifecycle operations:

1. **heartbeat()** (line ~134): Send heartbeat to broker/coordinator, receive lease offers
2. **pause()** (line ~143): Checkpoint any active sandboxes, notify broker
3. **withdraw()** (lines ~176-177): Checkpoint and terminate all active sandboxes, notify broker/coordinator of withdrawal

These are critical for Principle III (Donor Sovereignty) — donors must be able to pause and withdraw at any time with clean state transitions.

## Requirements

- Heartbeat: periodic signal to broker with node state, resource availability, active leases
- Pause: checkpoint all running sandboxes (SIGSTOP + snapshot), transition to Paused state, stop accepting new leases
- Withdraw: checkpoint all sandboxes, terminate them, wipe scoped working directory, revoke keypair, notify broker, leave no host state

## Success Criteria

- [ ] Heartbeat sends periodic updates to broker and receives lease offers
- [ ] Pause checkpoints active work and transitions agent to Paused state
- [ ] Withdraw fully cleans up: no files, processes, scheduled tasks, or network state remain
- [ ] State transitions follow the AgentState machine (Enrolling→Idle→Working→Paused→Withdrawing)
- [ ] Integration test: agent joins, receives work, pauses, resumes, withdraws cleanly
- [ ] `cargo test` passes with zero regressions

## Testing (Principle V)

- Run agent on real hardware, submit job, trigger pause, verify checkpoint saved
- Withdraw and verify zero host residue (scan for files, processes, network listeners)
- Test rapid pause/resume cycles under load
- Test withdraw during active job execution