feat: add Linq channel — real iMessage via API, no Mac required

Adds a complete Linq iMessage channel adapter that replaces the existing
iMessage channel's Mac Mini + dedicated Apple ID + SSH wrapper + Full Disk
Access setup with a single API key and phone number.

Core implementation (src/linq/):
- types.ts: Linq webhook event and message types
- accounts.ts: Multi-account resolution from config (env/file/inline token)
- send.ts: REST outbound via Linq Blue V3 API (messages, typing, reactions)
- probe.ts: Health check via GET /v3/phonenumbers
- monitor.ts: Webhook HTTP server with HMAC-SHA256 signature verification,
  replay protection, inbound debouncing, and full dispatch pipeline integration

Extension plugin (extensions/linq/):
- ChannelPlugin implementation with config, security, setup, outbound,
  gateway, and status adapters
- Supports direct and group chats, reactions, and media

Wiring:
- Channel registry, dock, config schema, plugin-sdk exports, and plugin
  runtime all updated to include the new linq channel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plugin-sdk/index.ts

@@ -122,6 +122,7 @@ export {
   DiscordConfigSchema,
   GoogleChatConfigSchema,
   IMessageConfigSchema,
+  LinqConfigSchema,
   MSTeamsConfigSchema,
   SignalConfigSchema,
   SlackConfigSchema,
@@ -450,5 +451,14 @@ export {
 } from "../line/markdown-to-line.js";
 export type { ProcessedLineMessage } from "../line/markdown-to-line.js";
 
+// Channel: Linq
+export {
+  listLinqAccountIds,
+  resolveDefaultLinqAccountId,
+  resolveLinqAccount,
+  type ResolvedLinqAccount,
+} from "../linq/accounts.js";
+export type { LinqProbe } from "../linq/types.js";
+
 // Media utilities
 export { loadWebMedia, type WebMediaResult } from "../web/media.js";