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/channels/registry.ts

@@ -13,6 +13,7 @@ export const CHAT_CHANNEL_ORDER = [
   "slack",
   "signal",
   "imessage",
+  "linq",
 ] as const;
 
 export type ChatChannelId = (typeof CHAT_CHANNEL_ORDER)[number];
@@ -109,6 +110,16 @@ const CHAT_CHANNEL_META: Record<ChatChannelId, ChannelMeta> = {
     blurb: "this is still a work in progress.",
     systemImage: "message.fill",
   },
+  linq: {
+    id: "linq",
+    label: "Linq",
+    selectionLabel: "Linq (iMessage API)",
+    detailLabel: "Linq iMessage",
+    docsPath: "/channels/linq",
+    docsLabel: "linq",
+    blurb: "real iMessage blue bubbles via API — no Mac required. Get a token at linqapp.com.",
+    systemImage: "bubble.left.and.text.bubble.right",
+  },
 };
 
 export const CHAT_CHANNEL_ALIASES: Record<string, ChatChannelId> = {
@@ -116,6 +127,7 @@ export const CHAT_CHANNEL_ALIASES: Record<string, ChatChannelId> = {
   "internet-relay-chat": "irc",
   "google-chat": "googlechat",
   gchat: "googlechat",
+  "linq-imessage": "linq",
 };
 
 const normalizeChannelKey = (raw?: string | null): string | undefined => {