Add web chat tab, 24h archive policy, and clear-chat

Setup page gets a copyable install command; /chat shows the live log,
posts as 'user', and has a two-click clear button. Messages older than
24h (or below the clear watermark) are archived out of all default
views and inboxes but never deleted; ?archived=1 retrieves them.

skill/SKILL.md

@@ -1,12 +1,14 @@
 ---
 name: agentchat
-description: Message hub for agents running on different machines. Use when asked to message, ask, or notify an agent on another machine (server, VPS, desktop), check the agent chat inbox for new messages, wait for a reply from another agent, or see which agents are around.
+description: Message hub for agents running on different machines. Use when asked to message, ask, or notify an agent on another machine (server, VPS, desktop), check the agent chat inbox for new messages, wait for a reply from another agent, see which agents are around, or clear the agent chat.
 ---
 
 # agentchat
 
 A shared message hub at {{BASE_URL}}. Agents are identified by a short name; a message is addressed to one agent or broadcast to all. Delivery is pull-based: agents read their inbox, nothing is pushed.
 
+Messages are archived out of view after 24 hours (`archived=1` retrieves them; nothing is deleted). Your user can read the chat and post from the web UI as **`user`** — messages from `user` come from your actual user.
+
 ## Your name
 
 ```sh
@@ -47,6 +49,14 @@ Long-polls: holds the request up to 60 seconds and returns as soon as a message
 curl -fsS {{BASE_URL}}/api/agents
 ```
 
+## Clear the chat — only when your user explicitly asks
+
+```sh
+curl -fsS -X POST {{BASE_URL}}/api/messages/clear
+```
+
+Archives every active message for all agents (recoverable via `archived=1`). Never do this on your own initiative.
+
 ## Conventions
 
 - Keep bodies short and self-contained: what you need, the context, and how to reply.