studio: agent window, a chat with a model that edits the scene

✦ Agent in the Studio's top bar opens a floating chat window. The model
(any OpenAI-compatible endpoint with tool calling; default B4L's inference
server, https://inference.p0.contact/v1) gets the 15 MCP tools plus two page
tools (select_object, set_view) and the Studio's state (selection, level,
rotation) in its system prompt. Answers and reasoning stream in; tool calls
show as rows with arguments and results; object ids link to the object;
Stop ends a turn; Undo takes the agent's edits back. The window moves by
its title bar and resizes at its top-left corner.

- studio/agent.js: shared by both front ends (waits for studio.js's
  isokit:studio event); MCP over /mcp or isokit.mcp in the web Studio
- isokit studio: /api/agent/{config,models,chat} relay the calls on their
  own threads (ureq, chunked and flushed per read), so the key stays on the
  machine and the endpoint needs no CORS; settings from ISOKIT_LLM_URL /
  _KEY / _MODEL or agent.json in the user's config folder
- web Studio: calls the endpoint from the page with a key kept in the
  browser; build_web.py --agent-url names a same-site relay as default; the
  deploy builds with /llm/v1, which deploy/Caddyfile forwards to the
  inference server (its nginx refuses the CORS preflight)
- tools/mock_llm.py + tools/agent_checks.py: the agent window checks in both
  check_studio.py and check_web.py

studio/index.html

@@ -68,11 +68,13 @@ document.documentElement.setAttribute("data-theme",t);})();
     <tr><th>Copy / delete</th><td>Ctrl+D / Delete</td></tr>
     <tr><th>Undo / redo</th><td>Ctrl+Z / Ctrl+Y</td></tr>
     <tr><th>View</th><td>drag the ground to pan, Ctrl + wheel or pinch to zoom, ↺ ↻ to rotate, levels to look inside</td></tr>
-    <tr><th>Agents</th><td>connect an AI agent over MCP; its edits appear here live (see the panel when nothing is selected)</td></tr>
+    <tr><th>Agent</th><td>✦ Agent opens a chat with a language model that edits the scene for you; Undo takes its steps back. Drag the window by its title, resize it at the top-left corner</td></tr>
+    <tr><th>MCP</th><td>connect an AI agent over MCP; its edits appear here live (see the panel when nothing is selected)</td></tr>
   </table>
   <form method="dialog"><button>Close</button></form>
 </dialog>
 <script src="/assets/viewer/isokit-view.js"></script>
 <script src="/assets/studio/studio.js"></script>
+<script src="/assets/studio/agent.js"></script>
 </body>
 </html>