feat: channels on the direct-IIOS path — kernel-client + KernelClientAdapter

- iios-kernel-client: RestClient.discoverThreads (GET /v1/threads/discover) +
  leaveThread (DELETE /v1/threads/:id/me); DiscoveredThread type
- KernelClientAdapter implements the four channel methods: browseChannels maps
  discovered public channels → ChannelSummary; createChannel → createThread
  (membership=channel, ADMIN, visibility/topic metadata); joinChannel →
  socket.openThread (governed public self-join); leaveChannel → rest.leaveThread
- adapter channel test over the fake transport (13 kernel-adapter tests); 57 green

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

packages/iios-messaging-ui/src/adapters/kernel-client.ts

@@ -8,6 +8,7 @@
 import { MessageSocket, RestClient } from '@insignia/iios-kernel-client';
 import type {
   AnnotationEvent,
+  DiscoveredThread,
   Message as KernelMessage,
   MessageEvents,
   OpenThreadResult,
@@ -17,7 +18,10 @@ import type {
 } from '@insignia/iios-kernel-client';
 import type { MessagingAdapter } from '../adapter';
 import type {
+  ChannelSummary,
+  ChannelVisibility,
   Conversation,
+  CreateChannelInput,
   Membership,
   Message,
   MessageEvent,
@@ -41,6 +45,8 @@ export interface RestPort {
   listThreads(filter?: { metadata?: Record<string, string> }): Promise<ThreadSummary[]>;
   createThread(opts: { membership?: string; creatorRole?: string; subject?: string; metadata?: Record<string, unknown> }): Promise<{ threadId: string }>;
   addParticipant(threadId: string, userId: string): Promise<void>;
+  discoverThreads(filter?: { metadata?: Record<string, string> }): Promise<DiscoveredThread[]>;
+  leaveThread(threadId: string): Promise<{ threadId: string; participantCount: number }>;
 }
 
 export interface KernelClientAdapterConfig {
@@ -165,6 +171,43 @@ export class KernelClientAdapter implements MessagingAdapter {
     await this.socket.markRead(threadId, messageId);
   }
 
+  // ── Channels ────────────────────────────────────────────────────
+  async browseChannels(): Promise<ChannelSummary[]> {
+    const found = await this.rest.discoverThreads({ metadata: { membership: 'channel', visibility: 'public' } });
+    return found.map((d) => {
+      const bag = (d.metadata as { topic?: string; visibility?: string } | null) ?? {};
+      const visibility: ChannelVisibility = bag.visibility === 'private' ? 'private' : 'public';
+      return {
+        threadId: d.threadId,
+        name: d.subject ?? 'channel',
+        topic: bag.topic ?? null,
+        visibility,
+        memberCount: d.participantCount,
+        joined: d.joined,
+      };
+    });
+  }
+
+  async createChannel(input: CreateChannelInput): Promise<{ threadId: string }> {
+    return this.rest.createThread({
+      membership: 'channel',
+      creatorRole: 'ADMIN',
+      subject: input.name,
+      metadata: { visibility: input.visibility, ...(input.topic ? { topic: input.topic } : {}) },
+    });
+  }
+
+  async joinChannel(threadId: string): Promise<void> {
+    // Self-join is a governed open_thread; OPA allows it for a public channel.
+    await this.socket.openThread(threadId);
+    this.joined.add(threadId);
+  }
+
+  async leaveChannel(threadId: string): Promise<void> {
+    await this.rest.leaveThread(threadId);
+    this.joined.delete(threadId);
+  }
+
   /** Detach socket handlers. Not part of the contract — call on teardown to avoid leaks. */
   close(): void {
     for (const off of this.offs) off();