feat(reports): classify git fetch as a git ref mutation (closes #297)

Reviewer reports treated 'git fetch prgs master' as read-only
diagnostics while claiming 'Git/worktree mutations: None'. Fetch edits
no files but updates remote-tracking refs, so hiding it under
diagnostics understates what the run mutated.

Add git_ref_mutating_commands() classifying fetch, pull, push, merge,
update-ref, remote update, branch -d/-D, and reset --hard as git ref
mutations, and assess_git_ref_mutation_report() which fails a report
when ref-updating commands ran but the report lacks a non-none 'Git ref
mutations' entry, omits 'fetched <remote>/<branch>' for a targeted
fetch, still claims 'Git/worktree mutations: None', or lists the
command under read-only diagnostics.

Tests cover fetch-only review, missing ledger entry, missing fetch
target, contradictory none-claim, read-only misclassification, worktree
creation (not a ref mutation), merge, cleanup branch deletion,
no-mutation blocked handoff, and dict command-log entries.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_git_ref_mutation_report.py

@@ -0,0 +1,101 @@
+"""Git ref mutations must be reported, never hidden as diagnostics (#297).
+
+``git fetch`` updates remote-tracking refs even though it edits no files.
+Reports that ran fetch (or any ref-updating command) must carry a
+``Git ref mutations`` entry and may not claim ``Git/worktree mutations:
+None`` or classify the fetch as read-only diagnostics.
+"""
+import sys
+import unittest
+
+sys.path.insert(0, str(__import__("pathlib").Path(__file__).resolve().parent.parent))
+
+import review_proofs
+
+
+def _assess(report, commands):
+    return review_proofs.assess_git_ref_mutation_report(
+        report, command_log=commands)
+
+
+class TestGitRefMutationDetection(unittest.TestCase):
+    def test_fetch_only_review_with_proper_ledger_passes(self):
+        report = "\n".join([
+            "Git ref mutations: fetched prgs/master",
+            "Review decision: APPROVE",
+        ])
+        res = _assess(report, ["git fetch prgs master"])
+        self.assertTrue(res["proven"], res["reasons"])
+        self.assertEqual(res["ref_mutating_commands"], ["git fetch prgs master"])
+
+    def test_fetch_without_ref_mutation_line_is_blocked(self):
+        report = "Review decision: APPROVE\nMutations: None\n"
+        res = _assess(report, ["git fetch prgs master"])
+        self.assertFalse(res["proven"])
+        self.assertTrue(
+            any("git ref mutations" in r.lower() for r in res["reasons"]))
+
+    def test_fetch_with_remote_branch_requires_fetched_target_in_report(self):
+        report = "Git ref mutations: some refs updated\n"
+        res = _assess(report, ["git fetch prgs master"])
+        self.assertFalse(res["proven"])
+        self.assertTrue(
+            any("prgs/master" in r for r in res["reasons"]))
+
+    def test_git_worktree_mutations_none_rejected_when_fetch_occurred(self):
+        report = "\n".join([
+            "Git ref mutations: fetched prgs/master",
+            "Git/worktree mutations: None",
+        ])
+        res = _assess(report, ["git fetch prgs master"])
+        self.assertFalse(res["proven"])
+        self.assertTrue(
+            any("git/worktree mutations" in r.lower() for r in res["reasons"]))
+
+    def test_fetch_classified_as_read_only_diagnostics_rejected(self):
+        report = "\n".join([
+            "Git ref mutations: fetched prgs/master",
+            "Read-only diagnostics: git fetch prgs master, git status",
+        ])
+        res = _assess(report, ["git fetch prgs master"])
+        self.assertFalse(res["proven"])
+        self.assertTrue(
+            any("read-only" in r.lower() for r in res["reasons"]))
+
+    def test_worktree_creation_is_not_a_ref_mutation(self):
+        report = "Worktree mutations: created branches/review-pr9\n"
+        res = _assess(report, ["git worktree add branches/review-pr9 prgs/master"])
+        self.assertTrue(res["proven"], res["reasons"])
+        self.assertEqual(res["ref_mutating_commands"], [])
+
+    def test_merge_command_counts_as_ref_mutation(self):
+        report = "Review decision: APPROVE\n"
+        res = _assess(report, ["git merge --no-ff feat/x"])
+        self.assertFalse(res["proven"])
+        self.assertIn("git merge --no-ff feat/x", res["ref_mutating_commands"])
+
+    def test_cleanup_branch_delete_counts_as_ref_mutation(self):
+        report = "Cleanup mutations: deleted branch feat/x\n"
+        res = _assess(report, ["git branch -d feat/x"])
+        self.assertFalse(res["proven"])
+        self.assertIn("git branch -d feat/x", res["ref_mutating_commands"])
+
+    def test_no_mutation_blocked_handoff_passes(self):
+        report = "\n".join([
+            "Git/worktree mutations: None",
+            "Current status: blocked handoff, no mutations performed",
+        ])
+        res = _assess(report, ["git status --porcelain", "git log --oneline -1"])
+        self.assertTrue(res["proven"], res["reasons"])
+        self.assertEqual(res["ref_mutating_commands"], [])
+
+    def test_command_log_dict_entries_supported(self):
+        report = "Git ref mutations: fetched prgs/master\n"
+        res = _assess(report, [{"command": "git fetch prgs master"}])
+        self.assertTrue(res["proven"], res["reasons"])
+        self.assertEqual(
+            res["ref_mutating_commands"], ["git fetch prgs master"])
+
+
+if __name__ == "__main__":
+    unittest.main()