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/files.rs

@@ -35,7 +35,10 @@ pub fn read_file(args: &ReadFileArgs) -> Outcome {
     };
 
     let mut buf = Vec::new();
-    let n = match file.take(MAX_READ as u64 + 1).read_to_end(&mut buf) {
+    let n = match file
+        .take(u64::try_from(MAX_READ).map_or(u64::MAX, |n| n.saturating_add(1)))
+        .read_to_end(&mut buf)
+    {
         Err(e) => return tool_err("read_file", path, &e.to_string()),
         Ok(n) => n,
     };