toolkit: no thread panic in http_fetch, no casts, exact egress-proxy form

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/toolkit/src/fetch.rs

@@ -80,7 +80,16 @@ pub fn fetch_with(curl: &Path, args: &HttpFetchArgs) -> Outcome {
         Some(r) => r,
         None => return Outcome::tool_error("http_fetch: curl has no standard error".to_string()),
     };
-    let stderr_handle = std::thread::spawn(move || read_capped(stderr_reader));
+    // `Builder`, not `spawn`, which panics when the system refuses a thread.
+    let stderr_handle = match std::thread::Builder::new().spawn(move || read_capped(stderr_reader))
+    {
+        Ok(handle) => handle,
+        Err(e) => {
+            let _ = child.kill();
+            let _ = child.wait();
+            return Outcome::tool_error(format!("http_fetch: cannot start a thread: {e}"));
+        }
+    };
 
     let mut body = Vec::new();
     let mut stdout = match child.stdout.take() {