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).

alembic/versions/014_drop_pm_approvals.py

@@ -0,0 +1,34 @@
+"""Drop unused Task column pm_approvals.
+
+Smoke run analysis (2026-05-12) initially flagged three Task fields
+as unused: pm_approvals, quick_context, proactive_context. A follow-up
+audit found quick_context and proactive_context are actively written
+(original_developer marker, doc_notes, PR creator tags; RAG context
+injection respectively). Only pm_approvals is truly orphaned — zero
+writers, only model + schema declarations as readers.
+
+Revision ID: 014_drop_pm_approvals
+Revises: 013_drop_role_enum
+Create Date: 2026-05-12
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "014_drop_pm_approvals"
+down_revision = "013_drop_role_enum"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.drop_column("tasks", "pm_approvals")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "tasks",
+        sa.Column("pm_approvals", sa.JSON, nullable=False, server_default="{}"),
+    )