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_postgres_backend.py

@@ -250,6 +250,25 @@ class AdapterFakeDriverTestCase(unittest.TestCase):
         self.assertIn("%s", fake.conn.cursor_obj.executed[0][0])
         self.assertNotIn("?", fake.conn.cursor_obj.executed[0][0])
 
+    def test_pragma_info_notnull_flag_matches_sqlite(self):
+        # PG 实证（2026-11-06）抓到的真 bug：适配层把
+        # information_schema.is_nullable 翻反了（NO -> 0），
+        # 与 sqlite PRAGMA table_info 的 notnull 语义相反。
+        # 此前无调用方读 row[3]，故只潜伏未炸；ensure 的
+        # outcome 放宽依赖该标志为真，此处钉住。
+        rows = [(1, "task_id", "text", "NO", None, 1),
+                (2, "outcome", "text", "YES", None, 0)]
+        fake = _FakeDriver(_FakeCursor(rows=rows))
+        db = _make_db(fake)
+        conn = db.connect()
+        info = conn.execute(
+            "PRAGMA table_info(task_attempts)").fetchall()
+        by_name = {row[1]: row for row in info}
+        self.assertEqual(by_name["task_id"][3], 1)
+        self.assertEqual(by_name["outcome"][3], 0)
+        self.assertEqual([row[1] for row in info],
+                         ["task_id", "outcome"])
+
     def test_unique_violation_maps_to_integrity_error(self):
         import sqlite3
         fake = _FakeDriver(_FakeCursor(fail_sequence=[_FakeUnique("dup")]))