fix: add timeout to subprocess calls in context_references (#3469)

_expand_git_reference() and _rg_files() called subprocess.run()
without a timeout. On a large repository, @diff, @staged, or
@git:N references could hang the agent indefinitely while git
or ripgrep processes slow output.

- Add timeout=30 to git subprocess in _expand_git_reference()
  with a user-friendly error message on TimeoutExpired
- Add timeout=10 to rg subprocess in _rg_files() returning
  None on timeout (falls back to os.walk folder listing)

Co-authored-by: memosr.eth <96793918+memosr@users.noreply.github.com>

agent/context_references.py

@@ -286,12 +286,16 @@ def _expand_git_reference(
     args: list[str],
     label: str,
 ) -> tuple[str | None, str | None]:
+    try:
         result = subprocess.run(
             ["git", *args],
             cwd=cwd,
             capture_output=True,
             text=True,
+            timeout=30,
         )
+    except subprocess.TimeoutExpired:
+        return f"{ref.raw}: git command timed out (30s)", None
     if result.returncode != 0:
         stderr = (result.stderr or "").strip() or "git command failed"
         return f"{ref.raw}: {stderr}", None
@@ -449,9 +453,12 @@ def _rg_files(path: Path, cwd: Path, limit: int) -> list[Path] | None:
             cwd=cwd,
             capture_output=True,
             text=True,
+            timeout=10,
         )
     except FileNotFoundError:
         return None
+    except subprocess.TimeoutExpired:
+        return None
     if result.returncode != 0:
         return None
     files = [Path(line.strip()) for line in result.stdout.splitlines() if line.strip()]