PG proof: fix task_attempts migration on real Postgres 16.15

- attempt_ledger.ensure: relax gap-3 outcome NOT NULL on Postgres
  (ALTER COLUMN outcome DROP NOT NULL, probe-gated); without it
  every resume write on a gap-3-migrated PG table fails with
  NotNullViolation. SQLite keeps NOT NULL (no such ALTER).
- db.PgCursor: PRAGMA table_info fake rows had the notnull flag
  inverted (NO -> 0); now matches sqlite semantics (1 = NOT NULL).
  No caller read row[3] before, so this was latent.
- ledger_onboarding: stale 'PG path unverified' notes updated to
  point at the real-PG proof.
- tests: pragma_info notnull pin (fake driver) + _column_notnull
  probe pins (fresh + gap-3 SQLite).

Proof: 28/28 checks on real Postgres 16.15 (see verification/
codex-task-attempts-pg-proof-20261106.md).

cloud/tests/test_task_attempts_unified_schema.py

@@ -30,6 +30,7 @@ from cloud.attempt_ledger import (  # noqa: E402
     SQLITE_DDL,
     TASK_ATTEMPTS_DDL,
     UNIFIED_COLUMNS,
+    _column_notnull,
     aggregate_attempts,
     ensure_task_attempts_table,
     read_attempt_rows,
@@ -133,6 +134,31 @@ class TestFreshAndMigrations(unittest.TestCase):
         self.assertEqual(self.conn.execute(
             "SELECT COUNT(*) FROM task_attempts").fetchone()[0], 1)
 
+    def test_column_notnull_probe(self):
+        # PG 实证（2026-11-06）：ensure 的 outcome 放宽依赖
+        # PRAGMA row[3] 的 notnull 语义；SQLite 侧钉住探针本身。
+        ensure_task_attempts_table(self.conn)
+        flags = _column_notnull(self.conn, "task_attempts")
+        self.assertFalse(flags["outcome"])
+        self.assertTrue(flags["task_id"])
+        self.assertTrue(flags["attempt_no"])
+        self.assertTrue(flags["is_defect"])
+
+    def test_column_notnull_probe_sees_gap3_outcome(self):
+        # gap-3 形态的 outcome 是 NOT NULL；探针必须报 True，
+        # 否则 PG 上的放宽不会触发，恢复写路仍会炸。
+        self.conn.execute(GAP3_DDL)
+        flags = _column_notnull(self.conn, "task_attempts")
+        self.assertTrue(flags["outcome"])
+        # SQLite 上放宽语句不合法、被 ensure 吞掉：迁移本身
+        # 不抛错、列集仍齐，但 outcome 保持 NOT NULL（需重建
+        # 表才能改，见批总结诚实边界；本批只修 PG 路径）。
+        ensure_task_attempts_table(self.conn)
+        self.assertEqual(set(_columns(self.conn)),
+                         set(UNIFIED_COLUMNS))
+        self.assertTrue(
+            _column_notnull(self.conn, "task_attempts")["outcome"])
+
     def test_old_shape_migrates_with_rows_intact(self):
         self.conn.execute(OLD_DDL)
         self.conn.execute(