refactor(v2): move channel adapters off v2 trunk

v2 ships with no channels baked in. All channel adapters (discord, slack,
telegram + helpers, whatsapp, whatsapp-cloud, gchat, github, imessage,
linear, matrix, resend, teams, webex) and their channel-specific setup
steps (pair-telegram, whatsapp-auth) now live on the `channels` branch
and get copied in via /add-*-v2 skills.

Removed:
- src/channels/{discord,slack,telegram*,whatsapp*,gchat,github,imessage,linear,matrix,resend,teams,webex}.ts
- setup/{pair-telegram,whatsapp-auth}.ts
- 14 channel-specific deps from package.json (@chat-adapter/*, @beeper/*,
  @bitbasti/*, @resend/chat-sdk-adapter, @whiskeysockets/baileys,
  chat-adapter-imessage, qrcode, @chat-adapter/state-memory unused)
- Their corresponding STEPS entries from setup/index.ts
- Channel imports from src/channels/index.ts

Kept:
- Channel infra: adapter.ts, channel-registry.ts (+ test), chat-sdk-bridge.ts,
  ask-question.ts, an empty-imports index.ts
- Chat SDK runtime (`chat`) for channels that copy in via Chat SDK bridge
- @chat-adapter/shared promoted from transitive to direct dep
  (channel-registry.ts uses NetworkError from it)

Verified: pnpm run build clean, 326 host tests pass.

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

package.json

@@ -22,27 +22,12 @@
     "test:watch": "vitest"
   },
   "dependencies": {
-    "@beeper/chat-adapter-matrix": "^0.2.0",
-    "@bitbasti/chat-adapter-webex": "^0.1.0",
-    "@chat-adapter/discord": "^4.24.0",
-    "@chat-adapter/gchat": "^4.24.0",
-    "@chat-adapter/github": "^4.24.0",
-    "@chat-adapter/linear": "^4.26.0",
-    "@chat-adapter/slack": "^4.24.0",
-    "@chat-adapter/state-memory": "^4.24.0",
-    "@chat-adapter/teams": "^4.24.0",
-    "@chat-adapter/telegram": "^4.24.0",
-    "@chat-adapter/whatsapp": "^4.24.0",
+    "@chat-adapter/shared": "^4.24.0",
     "@onecli-sh/sdk": "^0.3.1",
-    "@resend/chat-sdk-adapter": "^0.1.1",
-    "@types/qrcode": "^1.5.6",
-    "@whiskeysockets/baileys": "^6.17.16",
     "better-sqlite3": "11.10.0",
     "chat": "^4.24.0",
-    "chat-adapter-imessage": "^0.1.1",
     "cron-parser": "5.5.0",
-    "pino": "^9.6.0",
-    "qrcode": "^1.5.4"
+    "pino": "^9.6.0"
   },
   "devDependencies": {
     "@eslint/js": "^9.35.0",