Coop: lock combat actions for the full duration of an active run

Players in a co-op were getting their combat action back every midnight
reset and offered the dungeon option in the morning DM. Per spec they
should be "off in the Co-op" and unable to also solo-grind dungeons.

Fix at the DB layer (so all CanDoCombat() paths agree):
- New helper lockCoopCombatActions() sets combat_actions_used=99 for any
  user in an active coop run. Called immediately after every
  resetAllAdvDailyActions() — at startup and at midnight.
- Render layer: morning DM now shows "combat locked (in Co-op #N, day
  X/Y)" and replaces the dungeon section with an explanatory line
  pointing them at !coop status.

Combat returns to normal automatically once the run completes/wipes —
the next midnight reset finds status != 'active' and skips the lock.

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

internal/plugin/adventure.go

@@ -128,6 +128,9 @@ func (p *AdventurePlugin) Init() error {
 	if err := resetAllAdvDailyActions(); err != nil {
 		slog.Error("adventure: startup daily reset failed", "err", err)
 	}
+	if err := lockCoopCombatActions(); err != nil {
+		slog.Error("adventure: startup coop combat lock failed", "err", err)
+	}
 	// Revive any characters whose DeadUntil has expired
 	p.catchUpRespawns(chars)