Goal: add a simple heartbeat poll for the embedded agent that only notifies users when something matters, using the `HEARTBEAT_OK` sentinel. The heartbeat body we send is `HEARTBEAT` so the model can easily spot it.
- Extend the agent system prompt to explain: “If this is a heartbeat poll and nothing needs attention, reply exactly `HEARTBEAT_OK` and nothing else. For any alert, do **not** include `HEARTBEAT_OK`; just return the alert text.” Heartbeat prompt body is `HEARTBEAT`.
- New config key: `inbound.agent.heartbeatMinutes` (number of minutes; `0` disables).
- Default: 30 minutes.
- New optional idle override for heartbeats: `inbound.session.heartbeatIdleMinutes` (defaults to `idleMinutes`). Heartbeat skips do **not** update the session `updatedAt` so idle expiry still works.
- Each tick invokes the configured command with a short heartbeat body (e.g., “(heartbeat) summarize any important changes since last turn”) while reusing the active session args so Pi context stays warm.
- Heartbeats never create a new session implicitly: if there’s no stored session for the target (fallback path), the heartbeat is skipped instead of starting a fresh Pi session.
- Trim output. If the trimmed text equals `HEARTBEAT_OK` (case-sensitive) -> skip outbound message.
- Otherwise, send the text/media as normal, stripping the sentinel if it somehow appears.
- Treat empty output as `HEARTBEAT_OK` to avoid spurious pings.
## Logging requirements
- Normal mode: single info line per tick, e.g., `heartbeat: ok (skipped)` or `heartbeat: alert sent (32ms)`.
-`--verbose`: log start/end, command argv, duration, and whether it was skipped/sent/error; include session ID and connection/run IDs via `getChildLogger` for correlation.
- On command failure: warn-level one-liner in normal mode; verbose log includes stdout/stderr snippets.
## Failure/backoff
- If a heartbeat command errors, log it and retry on the next scheduled tick (no exponential backoff unless command repeatedly fails; keep it simple for now).
