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>

docs/gateway/heartbeat.md

@@ -162,6 +162,10 @@ If a `HEARTBEAT.md` file exists in the workspace, the default prompt tells the
 agent to read it. Think of it as your “heartbeat checklist”: small, stable, and
 safe to include every 30 minutes.
 
+If `HEARTBEAT.md` exists but is effectively empty (only blank lines and markdown
+headers like `# Heading`), Clawdbot skips the heartbeat run to save API calls.
+If the file is missing, the heartbeat still runs and the model decides what to do.
+
 Keep it tiny (short checklist or reminders) to avoid prompt bloat.
 
 Example `HEARTBEAT.md`: