Pin co-op invite posts; unpin on lock/cancel

Adds Base.PinEvent / Base.UnpinEvent helpers (m.room.pinned_events state)
that read-modify-write the existing pin list — idempotent on both sides.
The bot needs power level for state events; failures are logged but not
fatal.

Schema: add coop_dungeon_runs.invite_post_id to remember which event to
unpin. Migration entry included.

Wired:
- !coop start: pin the invite post in the games room
- !coop cancel: unpin before posting cancellation
- coopProcessLocks (auto-lock or auto-cancel): unpin before lock/expiry post

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

internal/plugin/plugin.go

@@ -37,6 +37,10 @@ type MessageContext struct {
 	Body      string
 	IsCommand bool // true if the message starts with the command prefix
 	Event     *event.Event
+	// OriginRoomID is set by plugins that rewrite RoomID for dispatch (e.g. holdem
+	// routes DM commands to the player's game room). When set, sender-private
+	// replies should go here instead of the rewritten RoomID.
+	OriginRoomID id.RoomID
 }
 
 // ReactionContext holds the context for a reaction event.
@@ -703,6 +707,61 @@ func (b *Base) SendDMID(userID id.UserID, text string) (id.EventID, error) {
 	return b.SendMessageID(roomID, text)
 }
 
+// PinEvent appends the given event ID to the room's m.room.pinned_events
+// state. No-op if already pinned. Requires the bot to have power level for
+// state events; failures are logged and returned without retry.
+func (b *Base) PinEvent(roomID id.RoomID, eventID id.EventID) error {
+	var content event.PinnedEventsEventContent
+	err := b.Client.StateEvent(context.Background(), roomID, event.StatePinnedEvents, "", &content)
+	if err != nil && !strings.Contains(err.Error(), "M_NOT_FOUND") {
+		slog.Error("pin: read state", "room", roomID, "err", err)
+		return err
+	}
+	for _, id := range content.Pinned {
+		if id == eventID {
+			return nil // already pinned
+		}
+	}
+	content.Pinned = append(content.Pinned, eventID)
+	_, err = b.Client.SendStateEvent(context.Background(), roomID, event.StatePinnedEvents, "", content)
+	if err != nil {
+		slog.Error("pin: write state", "room", roomID, "event", eventID, "err", err)
+	}
+	return err
+}
+
+// UnpinEvent removes the given event ID from the room's pinned events.
+// No-op if not currently pinned.
+func (b *Base) UnpinEvent(roomID id.RoomID, eventID id.EventID) error {
+	var content event.PinnedEventsEventContent
+	err := b.Client.StateEvent(context.Background(), roomID, event.StatePinnedEvents, "", &content)
+	if err != nil {
+		if strings.Contains(err.Error(), "M_NOT_FOUND") {
+			return nil
+		}
+		slog.Error("unpin: read state", "room", roomID, "err", err)
+		return err
+	}
+	out := content.Pinned[:0]
+	found := false
+	for _, id := range content.Pinned {
+		if id == eventID {
+			found = true
+			continue
+		}
+		out = append(out, id)
+	}
+	if !found {
+		return nil
+	}
+	content.Pinned = out
+	_, err = b.Client.SendStateEvent(context.Background(), roomID, event.StatePinnedEvents, "", content)
+	if err != nil {
+		slog.Error("unpin: write state", "room", roomID, "event", eventID, "err", err)
+	}
+	return err
+}
+
 // EditMessage edits an existing message using Matrix m.replace relation.
 func (b *Base) EditMessage(roomID id.RoomID, eventID id.EventID, newText string) error {
 	newContent := textContent(newText)