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/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "@clawdbot/tlon",
+  "version": "2026.1.22",
+  "type": "module",
+  "description": "Clawdbot Tlon/Urbit channel plugin",
+  "clawdbot": {
+    "extensions": [
+      "./index.ts"
+    ]
+  },
+  "dependencies": {
+    "@urbit/http-api": "^3.0.0",
+    "@urbit/aura": "^2.0.0",
+    "eventsource": "^2.0.2"
+  }
+}