fix(mcp): don't hand git the server's stdin in _git_head (#4440)

graph_stats asks git for HEAD to report staleness. The child inherited the
MCP stdio pipe, and on Windows git blocks on it: the call sat on the 3s
timeout, or hung under Claude Code. prs.py already isolates stdin (#3318);
these two _git_head copies were missed.

graphify/export.py

@@ -226,7 +226,8 @@ def _git_head(cwd: "str | Path | None" = None) -> str | None:
     import subprocess as _sp
     try:
         r = _sp.run(
-            ["git", "rev-parse", "HEAD"], capture_output=True, text=True, timeout=3,
+            ["git", "rev-parse", "HEAD"], stdin=_sp.DEVNULL,
+            capture_output=True, text=True, timeout=3,
             cwd=str(cwd) if cwd is not None else None,
         )
         return r.stdout.strip() if r.returncode == 0 else None

graphify/watch.py

@@ -376,7 +376,8 @@ def _git_head(cwd: Path | str | None = None) -> str | None:
     import subprocess as _sp
     try:
         r = _sp.run(
-            ["git", "rev-parse", "HEAD"], capture_output=True, text=True, timeout=3,
+            ["git", "rev-parse", "HEAD"], stdin=_sp.DEVNULL,
+            capture_output=True, text=True, timeout=3,
             cwd=str(cwd) if cwd is not None else None,
         )
         return r.stdout.strip() if r.returncode == 0 else None

tests/test_git_head_stdin.py

@@ -0,0 +1,21 @@
+"""_git_head must not hand git the caller's stdin (#4440).
+
+Under an MCP stdio host, stdin is the protocol pipe. On Windows a git child that
+inherits it blocks, so `graph_stats` stalls on the timeout or hangs. Same class
+as the prs.py isolation in #3318.
+"""
+import subprocess
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from graphify import export, watch
+
+
+@pytest.mark.parametrize("module", [export, watch], ids=["export", "watch"])
+def test_git_head_passes_devnull_stdin(module, tmp_path):
+    completed = MagicMock(returncode=0, stdout="abc123\n", stderr="")
+    with patch("subprocess.run", return_value=completed) as mock_run:
+        assert module._git_head(tmp_path) == "abc123"
+    _args, kwargs = mock_run.call_args
+    assert kwargs.get("stdin") == subprocess.DEVNULL