refactor(gateway): rename submit_for_qa to open_pr; pin atomic preconditions

Pre-fix, submit_for_qa opened a PR (side effect) and returned OK with
next='call i_am_done' — agents read the verb name, assumed they were
done with QA handoff, never called i_am_done, and PRs ended up
orphaned (PR #12 in the 2026-05-08 trace).

Two changes:

1. Rename submit_for_qa -> open_pr so the verb name matches the
   semantic. The PR opens here; the actual QA handoff happens at
   i_am_done. Renamed across:
   - choreographer/_impl.py (method)
   - mcp/flow_server.py (tool registration + _TOOLS dict)
   - api/routes/v2/flow_dev.py (route + handler)
   - api/schemas/v2/flow.py (OpenPrRequest)
   - services/gateway/verb_gates.py (_STATE_VERBS)
   - services/gateway/role_config.py (developer flow manifest)
   - services/gateway/content_actions.py (commit-success next= hint)
   - agent_sdk/server.py (post-tool guidance map)
   - runtime/orchestrator.py (developer prompt)
   - agents/prompts/{base,roles/developer,_generated/*}.md
   - tests/unit/gateway/test_submit_for_qa.py -> test_open_pr.py
   - tests/unit/api/routes/v2/test_flow_dev.py
   - tests/unit/gateway/test_verb_gates.py
   - tests/unit/api/test_correlation_id.py
   - tests/unit/mcp_servers/test_flow_server.py
   - tests/integration/test_full_lifecycle_real_db.py

2. New regression test (test_open_pr_does_not_create_pr_if_no_commits)
   pins the atomic invariant: preconditions (assignee, commits,
   no-prior-PR) must be checked BEFORE git.create_pr/push_branch run.
   Any future re-ordering breaks the test.

Tests: 3128 passing (3127 + 1 new), 100% coverage, ruff clean.

Note: TaskService.submit_for_qa() (the v1-layer service method) is
INTENTIONALLY not renamed — it's a different layer used by the v1
routes. The rename here is only the gateway verb surface.

tests/unit/gateway/test_choreographer_impl_branches.py

@@ -1049,3 +1049,38 @@ async def test_i_will_work_on_envelope_carries_introspection_on_rejection() -> N
     assert isinstance(body["valid_next_verbs"], list)
     # Lifecycle verbs are NOT in the list for a completed task.
     assert "i_will_work_on" not in body["valid_next_verbs"]
+
+
+@pytest.mark.asyncio
+async def test_open_pr_does_not_create_pr_if_no_commits() -> None:
+    """Atomic invariant: if commits[] is empty, open_pr must NOT call
+    git.create_pr. Pre-fix this was already true at the verb level, but
+    this test pins it as a regression: any future refactor that
+    re-orders precondition vs side effect breaks the test."""
+    dev_id = uuid4()
+    task_id = uuid4()
+    task = MagicMock(
+        status="in_progress",
+        assigned_to=dev_id,
+        commits=[],
+        pr_number=None,
+        branch_name="feature/backend/abc",
+        id=task_id,
+        title="t",
+        team="backend",
+        task_type="code",
+    )
+    task_svc = AsyncMock()
+    task_svc.get.return_value = task
+    task_svc.agent_for.return_value = MagicMock(role="developer", team="backend")
+    git_svc = AsyncMock()
+    git_svc.create_pr = AsyncMock()
+    git_svc.push_branch = AsyncMock()
+    deps = _make_deps(task=task_svc, git=git_svc)
+    c = Choreographer(deps)
+    env = await c.open_pr(dev_id, task_id)
+    body = env.as_dict()
+    assert body["error"] == "invalid_state"
+    assert "no commits" in body["message"]
+    git_svc.create_pr.assert_not_called()
+    git_svc.push_branch.assert_not_called()