ops: heartbeat v2 — programmatic health checks, zero tokens when healthy

Rewrote heartbeat.ts to perform all health checks in pure Node.js:
- Session liveness (alias + socket existence)
- Slack bridge responsiveness (HTTP 400 check)
- Stale worktrees (cross-ref with in-progress todos)
- Stuck todos (in-progress >2h with no dev-agent)
- Orphaned dev-agent sessions

Only injects an LLM prompt when something actually fails.
Saves ~$72/day on Opus vs the old approach that injected the
full HEARTBEAT.md checklist every 10 minutes regardless of health.

Updated tests to cover v2 functions (parseTodo, clampInt,
getExpectedSessions, etc.). All 50 tests pass.

Updated SKILL.md to document new heartbeat behavior.

Author: Baudbot