feat(templates): add heartbeat templates with emptiness detection and catchup

scripts/templates/heartbeat/README.md

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+# Heartbeat Scheduling
+
+Combines OpenClaw's HEARTBEAT.md task format with Paperclip's interval-based
+heartbeat model. Designed for Claude Code agents running on a schedule.
+
+## How it works
+
+1. A scheduler (cron/launchd/systemd) runs `heartbeat-runner.sh` at a fixed
+   interval (e.g., every 30 minutes)
+2. The runner reads `HEARTBEAT.md` for task definitions
+3. **Emptiness detection**: if the file has no real tasks, skip the API call
+   entirely (saves cost — from OpenClaw)
+4. For each task: check if it's due based on its interval and last-run time
+5. Run due tasks via `claude -p` with the task's prompt
+6. Suppress short acknowledgment responses (<300 chars containing HEARTBEAT_OK)
+7. Update `.heartbeat-state.json` with last-run timestamps
+
+## Two execution types (from OpenClaw)
+
+### systemEvent
+Injects a text event into an existing session. Lightweight, no new session.
+Use for: notifications, status checks, simple updates.
+Template: `scripts/templates/cron/system-event.sh`
+
+### agentTurn
+Fires a full agent turn with its own session. Full context, full tool access.
+Use for: background autonomous work, complex tasks, multi-step operations.
+Template: `scripts/templates/cron/agent-turn.sh`
+
+## Startup catchup (OpenClaw pattern)
+
+When the runner starts after downtime (e.g., machine was off):
+- Run `heartbeat-runner.sh --catchup`
+- Processes up to 5 missed tasks
+- 5-second stagger between tasks (prevents thundering herd)
+
+## Cost optimization
+
+- **Emptiness detection**: No API call if HEARTBEAT.md has no real content
+- **ackMaxChars suppression**: Responses under 300 chars with HEARTBEAT_OK
+  are logged but not displayed (saves downstream processing)
+- **Interval-based**: Only run tasks when actually due, not every heartbeat
+
+## Example cron entries
+
+```bash
+# Run heartbeat every 30 minutes
+*/30 * * * * /path/to/heartbeat-runner.sh >> /tmp/heartbeat.log 2>&1
+
+# Run heartbeat every hour with catchup on restart
+@reboot /path/to/heartbeat-runner.sh --catchup >> /tmp/heartbeat.log 2>&1
+0 * * * * /path/to/heartbeat-runner.sh >> /tmp/heartbeat.log 2>&1
+```
+
+## State file format
+
+`.heartbeat-state.json`:
+```json
+{
+  "email-check": { "last_run": 1712847600 },
+  "report-generation": { "last_run": 1712844000 }
+}
+```