feat(alembic): B2 drop unused pm_approvals Task column

Smoke run analysis initially flagged three Task fields as unused
(pm_approvals, quick_context, proactive_context). A follow-up audit
found quick_context (stores original_developer marker + doc notes +
PR creator + escalation notes) and proactive_context (RAG injection)
are actively used. Only pm_approvals is truly orphaned.

Migration 014 drops pm_approvals; downgrade() recreates it if ever
needed. The two false-positive fields stay untouched.

Spec ref: docs/superpowers/specs/2026-05-12-post-smoke-3-fixes-design.md
section B2 (re-scoped 2026-05-12).

tests/unit/api/test_schemas_tasks.py

@@ -276,7 +276,6 @@ def _stub_task(*, with_project: bool = False) -> SimpleNamespace:
         project=(SimpleNamespace(slug="proj-1") if with_project else None),
         docs_complete=False,
         pr_created=False,
-        pm_approvals={},
         team=Team.BACKEND,
         created_by=uuid4(),
         assigned_to=None,

tests/unit/services/test_audit_agent_id.py

@@ -259,7 +259,6 @@ async def test_submit_for_qa_writes_audit_with_dev_agent_id(
         pr_url="https://github.com/example/audit-test/pull/99",
         docs_complete=False,
         pr_created=True,
-        pm_approvals={},
         created_by=system_uuid,
         assigned_to=dev_uuid,
         claimed_by=dev_uuid,