fix(curator): stop the rollback safety snapshot from pruning its target

Rolling back to the oldest curator snapshot failed and deleted that
snapshot. rollback() takes a safety snapshot first, and snapshot_skills()
ends by pruning the backups directory down to keep (5 by default). At the
steady keep limit that prune removed the oldest snapshot, which is the very
one being restored, so the extract found no skills.tar.gz and the rollback
stopped with "snapshot extract failed (state restored)".

Thread an optional protect set through snapshot_skills() into _prune_old()
so the pre rollback safety snapshot can never evict the snapshot being
restored. Add two regression tests covering restore of the oldest snapshot
at the keep limit.

Fixes #47612

tests/agent/test_curator_backup.py

@@ -592,3 +592,62 @@ def test_restore_cron_skill_links_standalone(backup_env):
     assert report["restored"][0]["to"]["skills"] == ["narrow-a", "narrow-b"]
     assert len(report["skipped_missing"]) == 1
     assert report["skipped_missing"][0]["job_id"] == "job-gone"
+
+
+# ---------------------------------------------------------------------------
+# Rollback must not let the pre-rollback safety snapshot prune the target
+# (regression: restoring the oldest snapshot at the keep limit destroyed it)
+# ---------------------------------------------------------------------------
+
+def _three_ordered_snapshots(cb, skills, monkeypatch):
+    """Create snapshots 05-01 / 05-02 / 05-03 capturing growing trees, with
+    keep=3 so the backups dir is exactly at the retention limit. 05-01 holds
+    only 'pristine'; later snapshots add 'extra2' and 'extra3'. Leaves
+    _utc_id patched to a newest id so the rollback safety snapshot sorts
+    last. Returns the oldest snapshot id."""
+    monkeypatch.setattr(cb, "get_keep", lambda: 3)
+    plan = [
+        ("2026-05-01T00-00-00Z", ["pristine"]),
+        ("2026-05-02T00-00-00Z", ["pristine", "extra2"]),
+        ("2026-05-03T00-00-00Z", ["pristine", "extra2", "extra3"]),
+    ]
+    for snap_id, names in plan:
+        for n in names:
+            _write_skill(skills, n)
+        monkeypatch.setattr(cb, "_utc_id", lambda now=None, _i=snap_id: _i)
+        assert cb.snapshot_skills(reason=snap_id) is not None
+    monkeypatch.setattr(cb, "_utc_id", lambda now=None: "2026-05-09T00-00-00Z")
+    return "2026-05-01T00-00-00Z"
+
+
+def test_rollback_to_oldest_snapshot_at_keep_limit_succeeds(backup_env, monkeypatch):
+    """Restoring the oldest snapshot when the backups dir is at the keep limit
+    must succeed: the pre-rollback safety snapshot's prune step must not evict
+    the snapshot being restored."""
+    cb = backup_env["cb"]
+    skills = backup_env["skills"]
+    oldest = _three_ordered_snapshots(cb, skills, monkeypatch)
+
+    ok, msg, _ = cb.rollback(backup_id=oldest)
+
+    assert ok is True, f"rollback to oldest snapshot should succeed, got: {msg}"
+    # 05-01 only contained 'pristine'; a real restore reflects exactly that.
+    assert (skills / "pristine" / "SKILL.md").exists()
+    assert not (skills / "extra3").exists(), "tree was not restored to the oldest snapshot"
+
+
+def test_rollback_does_not_delete_the_snapshot_it_restores_from(backup_env, monkeypatch):
+    """The snapshot a rollback restores from must still exist afterwards — the
+    safety snapshot's prune must never delete the target."""
+    cb = backup_env["cb"]
+    skills = backup_env["skills"]
+    oldest = _three_ordered_snapshots(cb, skills, monkeypatch)
+    target_dir = skills / ".curator_backups" / oldest
+    assert target_dir.exists(), "precondition: target snapshot exists before rollback"
+
+    cb.rollback(backup_id=oldest)
+
+    assert target_dir.exists(), (
+        "the pre-rollback safety snapshot pruned away the snapshot being "
+        "restored — the oldest restore point is destroyed by restoring to it"
+    )