Fix compile warnings and tweak error message format

Warnings fixed:
- Remove unused 'warn' import from retry.rs
- Prefix unused 'output' param with underscore
- Prefix unused 'rel_start' with underscore
- Add #[allow(dead_code)] to G3Status::info()

Message format tweaked per feedback:
- 'g3: model overloaded [error]' (no attempt info)
- 'g3: retrying in 2.2s (1/3) ... [done]' (attempt info moved here)
- Handle empty error message in Status::Error to show just '[error]'

crates/g3-cli/src/completion.rs

@@ -224,7 +224,7 @@ impl Completer for G3Helper {
             let path: &str = &path_unescaped;
             
             // Complete just the path portion
-            let (rel_start, completions) = self.file_completer.complete(path, path.len(), ctx)?;
+            let (_rel_start, completions) = self.file_completer.complete(path, path.len(), ctx)?;
             
             if completions.is_empty() {
                 return Ok((pos, vec![]));