fix: align calendar API with actual HarborForge.Backend contract

Initial drop guessed the heartbeat shape; sim e2e against a running
harborforge-backend revealed the real contract is per-agent with
header auth, not server-wide with bearer:

  POST /calendar/agent/heartbeat
    headers: X-Agent-ID, X-Claw-Identifier
    body:    {claw_identifier, agent_id}
    response: {slots: [Slot], agent_status, message?}

  PATCH /calendar/slots/{id}/agent-update
  PATCH /calendar/slots/virtual/{vid}/agent-update
    body: {status, started_at?, actual_duration?}

  POST /calendar/agent/status
    body: {claw_identifier, agent_id, status}

Refactors:

  - internal/calendar/types.go now mirrors OpenclawPlugin/calendar/
    types.ts 1:1 (SlotStatus camelCase, real vs virtual slot id
    discrimination, event_data shape)
  - internal/calendar/bridge.go: header-based auth, per-agent method
    signatures, separate UpdateRealSlot vs UpdateVirtualSlot
  - internal/calendar/scheduler.go: per-agent heartbeat loop
    (one HTTP call per agent per tick), highest-priority slot
    selection, agent-update PATCH for terminal/non-terminal states
  - SingleActiveAgentID helper for main.bestEffortAgentID

Also fix two bugs found in sim:

  - bgCtx capture: AgentLister closures were capturing Init's ctx
    which dies the moment MCP initialize returns; switched to
    bgCtx (lifetime = plugin process)
  - tools.toolRestartStatus referenced a non-existent
    sch.RestartPending — HF backend has no restart endpoint per
    /openapi.json, so the tool now reports last_heartbeats freshness

Scheduler logs each tick + each heartbeat outcome at info so
operators can see backend connectivity without enabling debug.

E2E against http://harborforge-backend:8000 in sim:
  daemon → heartbeat → 404 "Agent not found"
  (= correct endpoint, correct headers, correct body — agent just
   isn't registered yet, which is expected for an untenanted
   plugin)

internal/tools/tools.go

@@ -176,11 +176,15 @@ func toolCalendarResume(ctx context.Context, deps Deps) (sdkplugin.ToolResult, e
 }
 
 func toolRestartStatus(deps Deps) (sdkplugin.ToolResult, error) {
+	// HarborForge backend doesn't expose a restart-pending endpoint
+	// (verified via /openapi.json) so we report the most recent
+	// heartbeat freshness instead. Useful for operators sanity-
+	// checking that the plugin's calendar loop is still alive.
 	sch := deps.Scheduler.Status()
 	return jsonResult(map[string]any{
-		"pending":        sch.RestartPending,
-		"last_heartbeat": sch.LastHeartbeat,
-		"observed_at":    time.Now().UTC(),
+		"pending":         false,
+		"last_heartbeats": sch.LastHeartbeats,
+		"observed_at":     time.Now().UTC(),
 	})
 }