feat(gateway): restore Gate Set E submit-qa field-level gates

i_am_done now strictly enforces the four pre-gateway field-level gates
restored from roboco/api/routes/tasks.py:903-940 at commit 254cc93:

- NOT_SELF_VERIFIED: task.self_verified must be true.
- NO_COMMITS: task.commits must be non-empty.
- NO_PR: task.pr_number must be set.
- NO_PROGRESS: task.progress_updates must have at least one entry.

Each missing field surfaces as a tracing_gap with the matching pre-
gateway error code in the missing list, and a remediation hint that
tells the dev exactly what to do.

The previous silent-catch-up behavior is preserved as a separate
opt-in verb i_am_done_with_catchup. Existing tests that asserted the
catch-up behavior have been migrated to the new verb.

This fixes the failure mode where a dev could call i_am_done with no
commits and the gateway would silently try to push nothing, open an
empty PR, etc. — now the dev sees an explicit error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/gateway/test_choreographer_reassignment.py

@@ -73,7 +73,8 @@ async def test_i_am_done_reassigns_task_to_qa_agent() -> None:
         acceptance_criteria_status=[
             {"criterion": "AC1", "referencing_artifact_id": "c1"}
         ],
-        commits=[],
+        # Gate Set E requires non-empty commits before submit_qa.
+        commits=[{"sha": "abc"}],
         documents=[],
         dev_notes="",
     )
@@ -124,7 +125,8 @@ async def test_i_am_done_skips_reassign_when_no_qa_agent() -> None:
         progress_updates=[{"message": "p"}],
         acceptance_criteria=[],
         acceptance_criteria_status=[],
-        commits=[],
+        # Gate Set E requires non-empty commits before submit_qa.
+        commits=[{"sha": "abc"}],
         documents=[],
         dev_notes="",
     )