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/README.md

@@ -0,0 +1,47 @@
+# fakechat
+
+Simple UI for testing the channel contract without an
+external service. Open a browser, type, messages go to your Claude Code
+session, replies come back.
+
+
+## Setup
+
+These are Claude Code commands — run `claude` to start a session first.
+
+Install the plugin:
+```
+/plugin install fakechat@claude-plugins-official
+```
+
+**Relaunch with the channel flag** — the server won't connect without this. Exit your session and start a new one:
+
+```sh
+claude --channels plugin:fakechat@claude-plugins-official
+```
+
+The server prints the URL to stderr on startup:
+
+```
+fakechat: http://localhost:8787
+```
+
+Open it. Type. The assistant replies in-thread.
+
+Set `FAKECHAT_PORT` to change the port.
+
+## Tools
+
+| Tool | Purpose |
+| --- | --- |
+| `reply` | Send to the UI. Takes `text`, optionally `reply_to` (message ID) and `files` (absolute path, 50MB). Attachment shows as `[filename]` under the text. |
+| `edit_message` | Edit a previously-sent message in place. |
+
+Inbound images/files save to `~/.claude/channels/fakechat/inbox/` and the path
+is included in the notification. Outbound files are copied to `outbox/` and
+served over HTTP.
+
+## Not a real channel
+
+There's no history, no search, no access.json, no skill. Single browser tab,
+fresh on every reload. This is a dev tool, not a messaging bridge.