feat(plugin): PLG-CAL-001 - define Calendar heartbeat request/response format

- Add plugin/calendar/types.ts: TypeScript interfaces for heartbeat
  request/response (CalendarHeartbeatRequest/Response, CalendarSlotResponse,
  SlotAgentUpdate, all enums: SlotType, SlotStatus, EventType)
- Add plugin/calendar/calendar-bridge.ts: CalendarBridgeClient HTTP client
  with heartbeat(), updateSlot(), updateVirtualSlot(), reportAgentStatus()
- Add plugin/calendar/index.ts: module entry point exporting all public types
- Add docs/PLG-CAL-001-calendar-heartbeat-format.md: full specification
  documenting claw_identifier and agent_id determination, request/response
  shapes, error handling, and endpoint summary
- Update plugin/openclaw.plugin.json: add calendarEnabled,
  calendarHeartbeatIntervalSec, calendarApiKey config options; clarify
  identifier description as claw_identifier

Refs: HarborForge.NEXT_WAVE_DEV_DIRECTION.md §6, BE-AGT-001

package-lock.json

@@ -0,0 +1,17 @@
+{
+  "name": "harbor-forge-openclaw-plugin",
+  "version": "0.2.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "harbor-forge-openclaw-plugin",
+      "version": "0.2.0",
+      "hasInstallScript": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    }
+  }
+}