feat(messaging-ui): channels — browse, join, leave, create (contract + mock + UI)

Adds a third membership beyond dm/group: a discoverable, joinable room.

- contract: Membership += 'channel'; Conversation.topic; ChannelSummary +
  CreateChannelInput; optional adapter methods browseChannels/createChannel/
  joinChannel/leaveChannel (capability-degrading — absent hides the UI)
- MockAdapter implements them (seeds a joined public, a joinable public, and a
  private channel); listConversations now returns only threads I'm in
- useChannels hook (browse/create/join/leave + supported flag)
- UI: sidebar sections (Channels vs Direct messages), a ＋ that opens a
  ChannelBrowser (join + create), # / lock glyphs; themeable styles
- KernelClientAdapter passes channel membership + topic through
- 4 channel tests (mock browse/join/create + render browse→join); 52 total green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/adapter.ts

@@ -1,6 +1,8 @@
 import type {
   Attachment,
+  ChannelSummary,
   Conversation,
+  CreateChannelInput,
   Membership,
   Message,
   MessageEvent,
@@ -53,4 +55,20 @@ export interface MessagingAdapter {
 
   /** Absent => treated as always connected (e.g. a pure-REST adapter). */
   isConnected?(): boolean;
+
+  // ── Channels (optional capability) ──────────────────────────────
+  // A channel is just a third membership beyond dm/group: a discoverable, joinable room.
+  // Implement all four to enable the channels UI; absent => the UI hides channels entirely.
+
+  /** Discoverable channels in the caller's scope, each flagged `joined`. */
+  browseChannels?(): Promise<ChannelSummary[]>;
+
+  /** Create a channel; the creator joins as admin. Returns the new thread id. */
+  createChannel?(input: CreateChannelInput): Promise<{ threadId: string }>;
+
+  /** Join a (public) channel by id. */
+  joinChannel?(threadId: string): Promise<void>;
+
+  /** Leave a channel by id. */
+  leaveChannel?(threadId: string): Promise<void>;
 }