feat(heartbeat): skip API calls when HEARTBEAT.md is effectively empty (#1535)

* feat: skip heartbeat API calls when HEARTBEAT.md is effectively empty

- Added isHeartbeatContentEffectivelyEmpty() to detect files with only headers/comments
- Modified runHeartbeatOnce() to check HEARTBEAT.md content before polling the LLM
- Returns early with 'empty-heartbeat-file' reason when no actionable tasks exist
- Preserves existing behavior when file is missing (lets LLM decide)
- Added comprehensive test coverage for empty file detection
- Saves API calls/costs when heartbeat file has no meaningful content

* chore: update HEARTBEAT.md template to be effectively empty by default

Changed instruction text to comment format so new workspaces benefit from
heartbeat optimization immediately. Users still get clear guidance on usage.

* fix: only treat markdown headers (# followed by space) as comments, not #TODO etc

* refactor: simplify regex per code review suggestion

* docs: clarify heartbeat empty file behavior (#1535) (thanks @JustYannicc)

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

CHANGELOG.md

@@ -11,6 +11,7 @@ Docs: https://docs.clawd.bot
 - CLI: add `clawdbot system` for system events + heartbeat controls; remove standalone `wake`.
 - Agents: add Bedrock auto-discovery defaults + config overrides. (#1553) Thanks @fal3.
 - Docs: add cron vs heartbeat decision guide (with Lobster workflow notes). (#1533) Thanks @JustYannicc.
+- Docs: clarify HEARTBEAT.md empty file skips heartbeats, missing file still runs. (#1535) Thanks @JustYannicc.
 - Markdown: add per-channel table conversion (bullets for Signal/WhatsApp, code blocks elsewhere). (#1495) Thanks @odysseus0.
 - Tlon: add Urbit channel plugin (DMs, group mentions, thread replies). (#1544) Thanks @wca4a.