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_scheduler.go

@@ -422,6 +422,13 @@ func (p *AdventurePlugin) midnightReset() error {
 		slog.Warn("adventure: daily action reset failed, retrying", "attempt", attempt+1, "err", resetErr)
 		time.Sleep(time.Duration(attempt+1) * 2 * time.Second)
 	}
+	if resetErr == nil {
+		// Re-lock combat for active co-op participants after the universal
+		// reset would otherwise have given them a fresh combat action.
+		if err := lockCoopCombatActions(); err != nil {
+			slog.Error("adventure: post-reset coop combat lock failed", "err", err)
+		}
+	}
 	if resetErr != nil {
 		return fmt.Errorf("reset daily actions after 3 attempts: %w", resetErr)
 	}