feat(update): syntax-validate critical files post-pull, auto-rollback on failure (#28669)

Catch the PR #28452 failure mode (orphan merge-conflict markers in
hermes_cli/config.py) on the user side: after git pull succeeds, compile
the files every 'hermes' invocation imports at startup. If any has a
syntax error, git reset --hard back to the pre-pull SHA so the install
stays bootable. User can retry once a fix lands upstream.

- New _capture_head_sha() + _validate_critical_files_syntax() helpers
- Wires both into _cmd_update_impl after the pull/reset succeeds
- Tests cover the helpers, the rollback flow, and a production-tree
  invariant (CI fails if main itself has a syntax error in a critical
  file — catches future broken commits before users hit them)

tests/hermes_cli/test_update_post_pull_syntax_guard.py

@@ -0,0 +1,153 @@
+"""Tests for the post-pull syntax guard in ``hermes update``.
+
+When a bad commit lands on ``main`` with a syntax error in a critical file
+(e.g. orphan merge-conflict markers in ``hermes_cli/config.py``), the CLI
+becomes unbootable — every ``hermes`` invocation imports those files at
+startup. The guard validates them after ``git pull`` and rolls back to the
+pre-pull SHA on failure so the user's install stays runnable.
+
+Reference incident: PR #28452 (May 18, 2026) shipped unresolved conflict
+markers in ``hermes_cli/config.py``; users who ran ``hermes update`` in
+the 7-minute window before #28458 landed could not run any ``hermes``
+command afterward.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from types import SimpleNamespace
+
+from hermes_cli import main as hermes_main
+
+
+# ---------------------------------------------------------------------------
+# _capture_head_sha
+# ---------------------------------------------------------------------------
+
+def test_capture_head_sha_returns_stripped_sha(monkeypatch, tmp_path):
+    def fake_run(cmd, **kwargs):
+        assert cmd[-2:] == ["rev-parse", "HEAD"]
+        return SimpleNamespace(stdout="deadbeefcafe\n", returncode=0)
+
+    monkeypatch.setattr(hermes_main.subprocess, "run", fake_run)
+
+    assert hermes_main._capture_head_sha(["git"], tmp_path) == "deadbeefcafe"
+
+
+def test_capture_head_sha_returns_none_on_git_failure(monkeypatch, tmp_path):
+    import subprocess as _sp
+
+    def fake_run(cmd, **kwargs):
+        raise _sp.CalledProcessError(returncode=128, cmd=cmd)
+
+    monkeypatch.setattr(hermes_main.subprocess, "run", fake_run)
+
+    assert hermes_main._capture_head_sha(["git"], tmp_path) is None
+
+
+def test_capture_head_sha_returns_none_on_empty_output(monkeypatch, tmp_path):
+    def fake_run(cmd, **kwargs):
+        return SimpleNamespace(stdout="\n", returncode=0)
+
+    monkeypatch.setattr(hermes_main.subprocess, "run", fake_run)
+
+    assert hermes_main._capture_head_sha(["git"], tmp_path) is None
+
+
+# ---------------------------------------------------------------------------
+# _validate_critical_files_syntax
+# ---------------------------------------------------------------------------
+
+def _populate_critical_tree(root: Path, *, broken_file: str | None = None) -> None:
+    """Create stub files for every entry in ``_UPDATE_CRITICAL_FILES``.
+
+    If ``broken_file`` is given, that file gets orphan merge-conflict markers
+    (the exact failure mode from PR #28452).
+    """
+    broken_payload = (
+        "x = {\n"
+        '    "a": 1,\n'
+        "<<<<<<< HEAD\n"
+        '    "b": 2,\n'
+        "=======\n"
+        '    "c": 0b6d673e7,\n'  # invalid binary literal — the actual error users saw
+        ">>>>>>> 0b6d673e7\n"
+        "}\n"
+    )
+    for relpath in hermes_main._UPDATE_CRITICAL_FILES:
+        path = root / relpath
+        path.parent.mkdir(parents=True, exist_ok=True)
+        if relpath == broken_file:
+            path.write_text(broken_payload)
+        else:
+            path.write_text("# stub\n")
+
+
+def test_validate_critical_files_syntax_ok_when_all_files_parse(tmp_path):
+    _populate_critical_tree(tmp_path)
+
+    ok, failing_path, error = hermes_main._validate_critical_files_syntax(tmp_path)
+
+    assert ok is True
+    assert failing_path is None
+    assert error is None
+
+
+def test_validate_critical_files_syntax_detects_conflict_markers(tmp_path):
+    """The exact PR #28452 failure mode: orphan ``<<<<<<<`` in config.py."""
+    _populate_critical_tree(tmp_path, broken_file="hermes_cli/config.py")
+
+    ok, failing_path, error = hermes_main._validate_critical_files_syntax(tmp_path)
+
+    assert ok is False
+    assert failing_path is not None and failing_path.endswith("hermes_cli/config.py")
+    assert error is not None
+    # The error mentions either the syntax error itself or the file path —
+    # either is enough proof we caught the bad commit.
+    assert "SyntaxError" in str(error) or "config.py" in str(error)
+
+
+def test_validate_critical_files_syntax_detects_break_in_main_py(tmp_path):
+    _populate_critical_tree(tmp_path, broken_file="hermes_cli/main.py")
+
+    ok, failing_path, _ = hermes_main._validate_critical_files_syntax(tmp_path)
+
+    assert ok is False
+    assert failing_path is not None and failing_path.endswith("hermes_cli/main.py")
+
+
+def test_validate_critical_files_syntax_tolerates_missing_files(tmp_path):
+    """A refactor may legitimately remove one of the critical files — the
+    guard should skip missing files, not falsely flag the install as broken."""
+    # Populate everything except hermes_constants.py
+    for relpath in hermes_main._UPDATE_CRITICAL_FILES:
+        if relpath == "hermes_constants.py":
+            continue
+        path = tmp_path / relpath
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text("# stub\n")
+
+    ok, failing_path, error = hermes_main._validate_critical_files_syntax(tmp_path)
+
+    assert ok is True
+    assert failing_path is None
+    assert error is None
+
+
+# ---------------------------------------------------------------------------
+# Repo invariant — the production tree itself must always pass the guard.
+# This catches the case where ``main`` ships a syntax error before the next
+# release; if a future ``hermes update`` would brick users, this test fails
+# in CI first.
+# ---------------------------------------------------------------------------
+
+def test_production_tree_passes_syntax_guard():
+    """The repo itself must always satisfy the guard the update command runs."""
+    repo_root = Path(__file__).resolve().parents[2]
+
+    ok, failing_path, error = hermes_main._validate_critical_files_syntax(repo_root)
+
+    assert ok is True, (
+        f"Critical-path file {failing_path} fails to parse on current main; "
+        f"hermes update would brick users. Error: {error}"
+    )