#12073: docs(heartbeat): clarify behavior when HEARTBEAT.md is missing

Fixes #11766. ## Why Docs previously implied that if `HEARTBEAT.md` is missing, heartbeats still run. In practice, workspace bootstrap may create an empty `HEARTBEAT.md`, and the existing “empty file = skip” rule then prevents heartbeat API calls. ## What changed - Clarified that **missing/empty** `HEARTBEAT.md` can result in **skip** due to bootstrap creating an empty file. - Added explicit workarounds: - `agents.defaults.skipBootstrap` to prevent bootstrap from creating the file - heartbeat `prompt` override options ## Repro (current behavior) 1. Remove `HEARTBEAT.md` 2. Run a heartbeat poll 3. If bootstrap creates an empty `HEARTBEAT.md`, the heartbeat will be skipped under the documented “empty = skip” rule Docs-only change (no code).