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

crates/isokit-project/src/assets.rs

@@ -11,6 +11,7 @@ pub const STUDIO_HTML: &str = include_str!("../../../studio/index.html");
 pub const STUDIO_JS: &str = include_str!("../../../studio/studio.js");
 pub const STUDIO_CSS: &str = include_str!("../../../studio/studio.css");
 pub const STUDIO_FAVICON: &str = include_str!("../../../studio/favicon.svg");
+pub const STUDIO_AGENT_JS: &str = include_str!("../../../studio/agent.js");
 
 /// Asset by its path in the repository ("viewer/isokit-view.js",
 /// "studio/studio.js"), from disk when `ISOKIT_ASSETS` is set.
@@ -28,6 +29,7 @@ pub fn get(path: &str) -> Option<Cow<'static, str>> {
         "studio/studio.js" => STUDIO_JS,
         "studio/studio.css" => STUDIO_CSS,
         "studio/favicon.svg" => STUDIO_FAVICON,
+        "studio/agent.js" => STUDIO_AGENT_JS,
         _ => return None,
     };
     Some(Cow::Borrowed(s))