refactor(v2): drop @chat-adapter/shared dep — duck-type NetworkError

The only use was channel-registry.ts checking `err instanceof NetworkError`
to retry transient setup failures. Switched to a duck-type predicate
(`err.name === 'NetworkError'`) so the dep is no longer needed at trunk
level. Channel skills bring it in transitively when they install their
Chat SDK adapter package.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/channels/channel-registry.ts

@@ -1,15 +1,21 @@
 /**
- * v2 Channel adapter registry.
+ * Channel adapter registry.
  *
  * Channels self-register on import. The host calls initChannelAdapters() at startup
  * to instantiate and set up all registered adapters.
  */
-import { NetworkError } from '@chat-adapter/shared';
 import type { ChannelAdapter, ChannelRegistration, ChannelSetup } from './adapter.js';
 import { log } from '../log.js';
 
 const SETUP_RETRY_DELAYS_MS = [2000, 5000, 10000];
 
+/** Duck-type check — adapters that throw an Error with `name === 'NetworkError'`
+ * (Chat SDK's `@chat-adapter/shared.NetworkError` and similar) get a retry on
+ * setup. Avoids depending on `@chat-adapter/shared` at trunk level. */
+function isNetworkError(err: unknown): err is Error {
+  return err instanceof Error && err.name === 'NetworkError';
+}
+
 const sleep = (ms: number) => new Promise<void>((resolve) => setTimeout(resolve, ms));
 
 const registry = new Map<string, ChannelRegistration>();
@@ -64,7 +70,7 @@ export async function initChannelAdapters(setupFn: (adapter: ChannelAdapter) =>
           await adapter.setup(setup);
           break;
         } catch (err) {
-          if (err instanceof NetworkError && attempt < SETUP_RETRY_DELAYS_MS.length) {
+          if (isNetworkError(err) && attempt < SETUP_RETRY_DELAYS_MS.length) {
             const delay = SETUP_RETRY_DELAYS_MS[attempt]!;
             log.warn('Channel adapter setup failed with network error, retrying', {
               channel: name,