feat(openclaw): add heartbeat toggle setting

[btc69m979y-dotcom]

Summary

• add a Settings > Agent Engine toggle for OpenClaw heartbeat, enabled by default
• persist the preference through Cowork config and sync it into agents.defaults.heartbeat.every
• write 1h when enabled and 0m when disabled, preserving the existing target: none, lightContext, and isolatedSession optimization

Notes

• OpenClaw documents agents.defaults.heartbeat.every: 0m as the disable path.
• The UI copy keeps the explanation focused on the 1-hour default cadence, token savings, and delayed/missing proactive background notifications when disabled.

Verification

• npm test -- coworkStore coworkSlice openclawConfigImpact openclawConfigSync.runtime
• npx eslint --ext ts,tsx --report-unused-disable-directives --max-warnings 0 src/main/coworkStore.ts src/main/coworkStore.test.ts src/main/main.ts src/main/preload.ts src/main/libs/openclawConfigImpact.ts src/main/libs/openclawConfigImpact.test.ts src/main/libs/openclawConfigSync.ts src/main/libs/openclawConfigSync.runtime.test.ts src/renderer/components/Settings.tsx src/renderer/services/i18n.ts src/renderer/store/slices/coworkSlice.ts src/renderer/store/slices/coworkSlice.test.ts src/renderer/types/cowork.ts src/renderer/types/electron.d.ts
• npm run compile:electron
• npm run build