The Session Resumption Designer

Design how this agent resumes an interrupted run.

Agent: [DESCRIBE]
Interruption causes: [PROCESS DEATH / USER PAUSE / TIMEOUT /
APPROVAL WAIT / RATE LIMIT]
Run length: [DESCRIBE]

Produce:
1. Checkpoint points: where state should be durably saved
2. What a checkpoint contains
3. What cannot be checkpointed and must be redone
4. Resumption: how the agent reconstructs context
5. What must be re-verified on resume — the world may have changed
6. Idempotency: how to avoid repeating a side-effecting action that
   already succeeded before the interruption
7. How the agent knows it was resumed rather than started fresh
8. How long a paused run stays resumable
9. What to tell the user on resumption

The idempotency question is the hard one. Address it specifically:
for each side-effecting tool, how does the agent know whether the
action completed before the interruption?