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/prisma/migrations/20260706194823_add_mention_inbox_kind/migration.sql

@@ -0,0 +1,2 @@
+-- AlterEnum
+ALTER TYPE "IiosInboxItemKind" ADD VALUE 'MENTION';

packages/iios-service/prisma/schema.prisma

@@ -79,6 +79,7 @@ enum IiosInboxItemKind {
   NEEDS_REPLY
   NEEDS_REVIEW
   NEEDS_APPROVAL
+  MENTION
   SUPPORT_UPDATE
   MEETING_FOLLOWUP
   DIGEST