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/adapters/mock.ts

@@ -1,7 +1,10 @@
 import type { MessagingAdapter } from '../adapter';
 import type {
   Attachment,
+  ChannelSummary,
+  ChannelVisibility,
   Conversation,
+  CreateChannelInput,
   Membership,
   Message,
   MessageEvent,
@@ -26,6 +29,8 @@ interface MockThread {
   subject: string | null;
   participants: string[];
   messages: Message[];
+  topic?: string | null;
+  visibility?: ChannelVisibility;
 }
 
 const nameById = new Map(MOCK_PEOPLE.map((p) => [p.id, p.name]));
@@ -66,6 +71,21 @@ export class MockAdapter implements MessagingAdapter {
         { id: 'm2', actorId: 'pp_dan', text: 'Crew is rolling out at 7.', at: seededAt, reactions: [] },
       ],
     });
+    // Channels: a joined public one, a joinable public one (I'm NOT in it → shows in browse only),
+    // and a private one I'm a member of.
+    this.threads.set('th_ch_general', {
+      threadId: 'th_ch_general', membership: 'channel', subject: 'general', topic: 'Company-wide chatter',
+      visibility: 'public', participants: [ME, 'pp_dan', 'pp_priya', 'pp_sofia'],
+      messages: [{ id: 'c1', actorId: 'pp_priya', text: 'Welcome to #general 👋', at: seededAt, reactions: [] }],
+    });
+    this.threads.set('th_ch_random', {
+      threadId: 'th_ch_random', membership: 'channel', subject: 'random', topic: 'Non-work banter',
+      visibility: 'public', participants: ['pp_dan', 'pp_sofia'], messages: [],
+    });
+    this.threads.set('th_ch_deals', {
+      threadId: 'th_ch_deals', membership: 'channel', subject: 'deals', topic: 'Big pipeline moves',
+      visibility: 'private', participants: [ME, 'pp_sofia'], messages: [],
+    });
   }
 
   currentActorId(): string {
@@ -73,19 +93,61 @@ export class MockAdapter implements MessagingAdapter {
   }
 
   async listConversations(): Promise<Conversation[]> {
-    return [...this.threads.values()].map((t) => {
-      const last = t.messages[t.messages.length - 1];
-      const others = t.participants.filter((p) => p !== ME);
-      return {
+    // Only threads I'm a member of — an un-joined public channel appears in browse, not here.
+    return [...this.threads.values()]
+      .filter((t) => t.participants.includes(ME))
+      .map((t) => {
+        const last = t.messages[t.messages.length - 1];
+        const others = t.participants.filter((p) => p !== ME);
+        return {
+          threadId: t.threadId,
+          title: t.subject || others.map((id) => nameById.get(id) ?? id).join(', ') || 'Conversation',
+          subject: t.subject,
+          membership: t.membership,
+          participants: [...t.participants],
+          unread: 0,
+          ...(t.topic != null ? { topic: t.topic } : {}),
+          ...(last ? { lastMessage: last.text, lastAt: last.at } : {}),
+        };
+      });
+  }
+
+  async browseChannels(): Promise<ChannelSummary[]> {
+    // Public channels are discoverable; private ones only if I'm already a member.
+    return [...this.threads.values()]
+      .filter((t) => t.membership === 'channel' && (t.visibility === 'public' || t.participants.includes(ME)))
+      .map((t) => ({
         threadId: t.threadId,
-        title: t.subject || others.map((id) => nameById.get(id) ?? id).join(', ') || 'Conversation',
-        subject: t.subject,
-        membership: t.membership,
-        participants: [...t.participants],
-        unread: 0,
-        ...(last ? { lastMessage: last.text, lastAt: last.at } : {}),
-      };
+        name: t.subject ?? 'channel',
+        topic: t.topic ?? null,
+        visibility: t.visibility ?? 'public',
+        memberCount: t.participants.length,
+        joined: t.participants.includes(ME),
+      }));
+  }
+
+  async createChannel(input: CreateChannelInput): Promise<{ threadId: string }> {
+    const threadId = `th_ch_${this.seq++}`;
+    this.threads.set(threadId, {
+      threadId,
+      membership: 'channel',
+      subject: input.name,
+      topic: input.topic ?? null,
+      visibility: input.visibility,
+      participants: [ME],
+      messages: [],
     });
+    return { threadId };
+  }
+
+  async joinChannel(threadId: string): Promise<void> {
+    const t = this.threads.get(threadId);
+    if (t && !t.participants.includes(ME)) t.participants = [...t.participants, ME];
+  }
+
+  async leaveChannel(threadId: string): Promise<void> {
+    const t = this.threads.get(threadId);
+    if (t) t.participants = t.participants.filter((p) => p !== ME);
   }
 
   async openThread(p: { participantIds: string[]; membership?: Membership; subject?: string }): Promise<{ threadId: string }> {