fix(gateway): gate review diffs against the task's real parent branch (#444) (#454)

The in-path PR-review gate's evidence diff (claim_gate_review) and the
pr_pass conventions guard derived their diff base via parent_branch_for
string surgery, which reuses the child branch's own team segment — wrong
for every cross-team hop (a frontend child of a main_pm root derives a
ref that never existed) and silently falls back to the repo default
branch, so the reviewer judged the entire inherited base-branch content
as the task's own work and failed acceptance criteria the task never
touched. Bounced a live goals-tab fix three times, unfixable by branch
surgery.

The gate now resolves the base via resolve_parent_branch (the parent
task's recorded branch_name, cross-team correct) and threads it as a new
preferred_parent override through git.diff / list_changed_files /
conventions_check_for_task — consulted only when no explicit base is
given, so the pinned literal-base contract (base="HEAD~1") and every
other diff caller (QA, doc, content) are byte-identical. Parent lookup
fails open (derived-base fallback) like the other resolve_parent_branch
call sites, and is skipped entirely while the conventions flag is off.

Also excludes .uv-cache/ and .claude/ (agent worktrees, private uv
cache) from the markdown prose scanner — both are repo-local tool dirs
whose vendored/generated files tripped make reflow-check.

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/services/test_git_conventions_check_fail_closed.py

@@ -95,6 +95,28 @@ async def test_no_changed_files_still_fails_open() -> None:
     assert result["findings"] == []
 
 
+@pytest.mark.asyncio
+async def test_preferred_parent_forwards_to_list_changed_files() -> None:
+    """The in-path PR-review gate's cross-team parent (see ``diff``) must
+    reach ``list_changed_files`` so the validator never analyzes files
+    inherited from the wrong-team derived base."""
+    svc = _service()
+    _bind(svc, "_workspace_for_branch", AsyncMock(return_value=Path("/tmp/ws")))
+    changed = AsyncMock(return_value=[])
+    _bind(svc, "list_changed_files", changed)
+    actor_id = uuid4()
+    await svc.conventions_check_for_task(
+        actor_id,
+        _task("feature/frontend/root--cell"),
+        preferred_parent="feature/main_pm/root",
+    )
+    changed.assert_awaited_once_with(
+        branch_name="feature/frontend/root--cell",
+        actor_agent_id=actor_id,
+        preferred_parent="feature/main_pm/root",
+    )
+
+
 @pytest.mark.asyncio
 async def test_validator_timeout_fails_closed_and_reaps(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch