fix(gateway): evidence/QA/doc paths populate files_changed from git

Bug:
    ContentActions.evidence() hard-coded files_changed=[] and diffed
    against HEAD~1 instead of the branch's parent. QA's _build_qa_claim_
    evidence (and doc/_impl mirrors) sourced files_changed from
    work_session.files_modified, which the gateway commit() never
    populates (no add_files_modified plumbing). Result: QA / docs / PM
    reviewers saw an empty change list on real PRs and only the latest
    commit's delta — flagged in smoke-9 when PR #20 showed the README
    change on GitHub but evidence() reported empty.

Fix:
    Added GitService.list_changed_files (git diff --name-only against
    parent branch). evidence(), _build_qa_claim_evidence,
    _claim_doc_evidence, and _build_i_am_done_ok all source files_changed
    from this — git is the authoritative source. evidence() also drops
    the HEAD~1 base so the diff is the full PR.

    Wired EvidenceRepo into ContentActionsDeps so evidence() returns
    journal_highlights too, matching the QA/doc shape.

tests/unit/gateway/test_content_actions_ownership.py

@@ -44,6 +44,11 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
     journal = overrides.get("journal", AsyncMock())
     workspace = overrides.get("workspace", AsyncMock())
     notifications = overrides.get("notifications", AsyncMock())
+    if "evidence_repo" in overrides:
+        evidence_repo = overrides["evidence_repo"]
+    else:
+        evidence_repo = AsyncMock()
+        evidence_repo.journal_highlights_for_task.return_value = []
     return ContentActionsDeps(
         task=task,
         git=git,
@@ -52,6 +57,7 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
         journal=journal,
         workspace=workspace,
         notifications=notifications,
+        evidence_repo=evidence_repo,
     )