fix(heartbeat): honor top-level heartbeat.enabled=false setting

[ZVNC28]

Description

The Settings type declared heartbeat.enabled only at the per-agent level (AgentConfig.heartbeat.enabled). A top-level heartbeat.enabled flag in settings.json was silently ignored, and startHeartbeat() was called unconditionally in packages/main/src/index.ts:272, so users had no way to disable the heartbeat interval globally without editing every agent entry.

This came up in practice running an 8-agent team against a single shared inference backend: every hour, all 8 heartbeats fire concurrently and serialize through the same GPU, creating a large backlog. The natural workaround — heartbeat: { enabled: false } at the top of settings.json — compiled fine (TypeScript didn't complain because the field wasn't in the type) but did nothing at runtime.

Changes

• Add optional top-level heartbeat?: { enabled?: boolean } to the Settings interface in packages/core/src/types.ts
• Add isHeartbeatEnabled() helper in packages/main/src/heartbeat.ts
• Short-circuit startHeartbeat() when disabled, logging a clear "Heartbeat disabled (heartbeat.enabled=false in settings)" message instead of silently still creating the interval
• Also check in tick() as a safety net, so runtime config changes take effect without a daemon restart

Default behavior is unchanged — when the flag is undefined (the common case), heartbeat continues to run exactly as before. Per-agent agent.heartbeat.enabled remains fully supported.

Testing

• npm run build — clean compile, no new warnings
• Verified locally that heartbeat: { enabled: false } at the top level now logs "Heartbeat disabled" on daemon start and the interval is never created

Checklist

• PR title follows conventional commit format
• I have tested these changes locally
• My changes don't introduce new warnings or errors
• I have updated documentation if needed — no existing docs mention heartbeat.enabled; happy to add a note to the README if you'd prefer