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

.gitea/workflows/deploy-web.yml

@@ -78,7 +78,9 @@ jobs:
 
       - name: Build dist/web
         run: |
-          python3 tools/build_web.py
+          # the agent window's default endpoint: the site's relay to B4L's
+          # inference server (deploy/Caddyfile, handle_path /llm/*)
+          python3 tools/build_web.py --agent-url /llm/v1
           echo "$GITHUB_SHA" > dist/web/version.txt
 
       - name: Set up SSH