feat(calendar): GET /agent/status — read-only status query for plugin gate

Previously only POST /agent/status existed (for state transitions).
Fabric.OpenclawPlugin's triage on-call gate needs to check whether
the on-duty agent is currently on_call without flipping their state —
so the wake decision is read-only. GET returns {agent_id, status},
404 if unknown.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/api/routers/calendar.py

@@ -561,6 +561,29 @@ def agent_update_virtual_slot(
     return TimeSlotEditResponse(slot=_slot_to_response(slot), warnings=[])
 
 
+@router.get(
+    "/agent/status",
+    summary="Read an agent's current runtime status (no side effects)",
+)
+def get_agent_status(
+    agent_id: str = Query(..., description="Target agent_id"),
+    x_claw_identifier: str = Header(..., alias="X-Claw-Identifier"),
+    db: Session = Depends(get_db),
+):
+    """Return `{agent_id, status}` so callers (Fabric.OpenclawPlugin's
+    triage on-call gate, etc.) can decide whether the agent is currently
+    eligible without flipping their state.
+
+    No-op for unknown agents — returns 404 with `{detail: 'Agent not
+    found'}` so the caller can decide whether to fail-open or fail-closed.
+    """
+    agent = _require_agent(db, agent_id, x_claw_identifier)
+    return {
+        "agent_id": agent.agent_id,
+        "status": agent.status.value if hasattr(agent.status, 'value') else str(agent.status),
+    }
+
+
 @router.post(
     "/agent/status",
     summary="Update agent runtime status from plugin",