Add Tlon/Urbit channel plugin

Adds built-in Tlon (Urbit) channel plugin to support decentralized messaging on the Urbit network.

Features:
- DM and group chat support
- SSE-based real-time message monitoring
- Auto-discovery of group channels
- Thread replies and reactions
- Integration with Urbit's HTTP API

This resolves cron delivery issues with external Tlon plugins by making it a first-class built-in channel alongside Telegram, Signal, and other messaging platforms.

Implementation includes:
- Plugin registration via ClawdbotPluginApi
- Outbound delivery with sendText and sendMedia
- Gateway adapter for inbound message handling
- Urbit SSE client for event streaming
- Core bridge for Clawdbot runtime integration

Co-authored-by: William Arzt <william@arzt.co>

extensions/tlon/index.ts

@@ -0,0 +1,16 @@
+import type { ClawdbotPluginApi } from "clawdbot/plugin-sdk";
+import { emptyPluginConfigSchema } from "clawdbot/plugin-sdk";
+
+import { tlonPlugin } from "./src/channel.js";
+
+const plugin = {
+  id: "tlon",
+  name: "Tlon",
+  description: "Tlon/Urbit channel plugin",
+  configSchema: emptyPluginConfigSchema(),
+  register(api: ClawdbotPluginApi) {
+    api.registerChannel({ plugin: tlonPlugin });
+  },
+};
+
+export default plugin;