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/integration/test_lifecycle_real_db.py

@@ -121,6 +121,12 @@ class _StubGit:
         del branch_name, base, actor_agent_id
         return "stub diff"
 
+    async def list_changed_files(
+        self, *, branch_name: str, base: Any = None, actor_agent_id: Any = None
+    ) -> list[str]:
+        del branch_name, base, actor_agent_id
+        return []
+
     async def pr_target(self, pr_number: int, *, actor_agent_id: Any = None) -> str:
         del pr_number, actor_agent_id
         return "main"