Heartbeat timer bucket receipts and restart dedupe

[davidahmann]

Problem observed
The heartbeat scheduler did not have a persisted per-interval identity for timer wakes, so replaying the same elapsed interval after restart depended on transient active-run state instead of a durable contract.

Why it matters operationally
Paperclip positions heartbeats as the autonomous execution path for agents. When restart timing and queue timing interact badly, duplicate receipts or duplicate wake attempts erode operator trust because the board can no longer tell whether a second run represents a new interval or a replay of the same one. That weakens the audit trail precisely where the product claims governance and deterministic operational evidence.

Minimal repro

1. Configure an agent with heartbeat enabled and an elapsed interval.
2. Trigger the scheduler for that interval.
3. Replay the same timer interval before the happy-path heartbeat timestamp advances, or after a restart replays the same elapsed interval.
4. Observe that the timer path previously had no persisted bucket identity to distinguish “same interval replay” from “new interval.”

Fix approach
I threaded a stable timer-bucket key through the timer wake payload/context and used that persisted key to short-circuit duplicate timer buckets against existing wakeup receipts. If the original timer bucket is still active, the duplicate request is coalesced onto the existing run. If the bucket already completed, the replay records an explicit skipped receipt instead of creating fresh work.

Validation evidence

• pnpm test:run server/src/__tests__/heartbeat-workspace-session.test.ts
• pnpm --filter @paperclipai/server typecheck
• Added focused unit coverage for deterministic timer-bucket generation

Maintainer question: would you want the duplicate-bucket skip reason promoted into the operator-visible activity surface as a first-class governance receipt as well?
Inspired by research context: CAISI publishes independent, reproducible AI agent governance research: https://caisi.dev