fix(mcp): list_maintenance_plans reports only a slot that fired as last_slot_at (#5863)

A new plan's last_slot_at holds the moment it was made, so its first
slot is the next one; the tool returned that moment, a Wednesday
evening for a Sunday plan, beside latest_run: null. cadence.fired_slot
returns it only when it falls on one of the plan's slots.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/cadence.py

@@ -55,6 +55,16 @@ def last_slot(plan, now: datetime) -> datetime:
     return slot.astimezone(timezone.utc)
 
 
+def fired_slot(plan) -> datetime | None:
+    """The last slot that fired, or None while none has. A new plan's
+    last_slot_at holds the moment it was made, so its first slot is the
+    next one rather than one already past; that moment is no slot (#5863)."""
+    at = plan.last_slot_at
+    if at is None:
+        return None
+    return at if last_slot(plan, at) == at else None
+
+
 def next_slot(plan, now: datetime) -> datetime:
     return last_slot(plan, now) + timedelta(days=7)