Add fakechat channel plugin (#738)

Localhost web chat UI for testing the channel notification flow.
No tokens, no access control, no third-party service. Serves an
iMessage-style UI on localhost; messages posted there arrive as
channel notifications, replies render in the UI.

Useful for developing against the channel protocol without a live
messaging account.

Ships full source — server.ts runs locally via bun, started by the
.mcp.json command.

external_plugins/fakechat/.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "fakechat": {
+      "command": "bun",
+      "args": ["run", "--cwd", "${CLAUDE_PLUGIN_ROOT}", "--shell=bun", "--silent", "start"]
+    }
+  }
+}