feat(iios): @mentions via inbox fan-out + my-annotations query (pins/saves)

Mentions ride the existing event-driven inbox — no chat parsing in the kernel:
- send() carries an OPAQUE mentions[] (userIds) into the message event; the kernel
  never parses "@". The app supplies the notify-list.
- InboxProjector fans out a MENTION inbox item (new generic inbox kind) to each
  mentioned *participant* (never the sender), idempotent per source message; reading
  the thread resolves the reader's MENTION + NEEDS_REPLY items to DONE.
- New MENTION value in the generic IiosInboxItemKind taxonomy (migration).

Pins/saves reuse the annotation primitive; new generic query powers a Saved list:
- MessageService.listMyAnnotated(principal, type) + GET /v1/threads/my-annotations
  returns the caller's annotated messages (type "save") with thread context.

Tests: 182 pass (+4: mention fan-out, resolve-on-read, saved query, opaque mentions).
Verified live: mention→inbox delivery, read→resolve, pin/save persistence.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/src/messaging/message.gateway.ts

@@ -91,7 +91,7 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection {
   @SubscribeMessage('send_message')
   async sendMessage(
     @ConnectedSocket() client: Socket,
-    @MessageBody() body: { threadId: string; content: string; contentRef?: string; parentInteractionId?: string },
+    @MessageBody() body: { threadId: string; content: string; contentRef?: string; parentInteractionId?: string; mentions?: string[] },
   ) {
     const { principal } = client.data as SocketState;
     const msg = await this.messages.send(
@@ -101,6 +101,7 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection {
       randomUUID(),
       undefined,
       body.parentInteractionId,
+      body.mentions,
     );
     this.emittedLocally.add(msg.id);
     this.server.to(body.threadId).emit('message', msg);