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/db.py

@@ -733,7 +733,7 @@ class PgCursor:
                     info_cur.close()
                 self._fake_rows = [
                     PgRow((i, r[1], r[2],
-                           0 if r[3] == "NO" else 1, r[4], r[5]),
+                           1 if r[3] == "NO" else 0, r[4], r[5]),
                           ("cid", "name", "type", "notnull",
                            "dflt_value", "pk"))
                     for i, r in enumerate(raw)]