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.

agents/prompts/_generated/verbs.md

@@ -15,7 +15,7 @@ as the source of truth for verb signatures.
 |------|-------------|
 | `give_me_work` | `give_me_work()` |
 | `i_will_work_on` | `i_will_work_on(task_id: UUID, plan: str | None = None)` |
-| `submit_for_qa` | `submit_for_qa(task_id: UUID)` |
+| `open_pr` | `open_pr(task_id: UUID)` |
 | `i_am_done` | `i_am_done(task_id: UUID, notes: str = '')` |
 | `i_am_blocked` | `i_am_blocked(task_id: UUID, reason: str)` |
 | `unclaim` | `unclaim(task_id: UUID)` |