fix: surface actual git clone error in build logs

CalledProcessError only shows exit code, not stderr. Log the real
error message (e.g. permission denied, auth failure) to make
debugging clone failures straightforward.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AbirAbbas

swe_af/app.py

@@ -79,12 +79,15 @@ async def build(
     if cfg.repo_url and not os.path.exists(os.path.join(repo_path, ".git")):
         app.note(f"Cloning {cfg.repo_url} → {repo_path}", tags=["build", "clone"])
         os.makedirs(repo_path, exist_ok=True)
-        subprocess.run(
+        clone_result = subprocess.run(
             ["git", "clone", cfg.repo_url, repo_path],
-            check=True,
             capture_output=True,
             text=True,
         )
+        if clone_result.returncode != 0:
+            err = clone_result.stderr.strip()
+            app.note(f"Clone failed (exit {clone_result.returncode}): {err}", tags=["build", "clone", "error"])
+            raise RuntimeError(f"git clone failed (exit {clone_result.returncode}): {err}")
     else:
         # Ensure repo_path exists even when no repo_url is provided (fresh init case)
         # This is needed because planning agents may need to read the repo in parallel with git_init