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

web/web.js

@@ -286,7 +286,7 @@ function backend() {
     agentBox: function () {
       var dir = isFolder() ? "<folder of " + P.rec.name + ">" : "<project folder>";
       return '<div class="st-box"><h2>AI agents (MCP)</h2>' +
-        '<p class="st-help">The 15 isokit MCP tools run inside this page: browsers with WebMCP offer them to their assistant, and scripts call <b>isokit.mcp(message)</b>.</p>' +
+        '<p class="st-help">The 15 isokit MCP tools run inside this page: ✦&nbsp;Agent (top bar) chats with a model that uses them, browsers with WebMCP offer them to their assistant, and scripts call <b>isokit.mcp(message)</b>.</p>' +
         (isFolder() ? "Desktop agents work on the same folder; their edits show up here live:" : "For desktop agents, open a folder (Projects → Open folder) or export the project, then:") +
         "<code>isokit mcp " + esc(dir) + "</code></div>";
     }