Fix op checkpoint JSONB binding

[uncfreak1255-code]

Summary

• Fix recordCompleted so op_checkpoints.completed_keys is written as a real JSONB array on Postgres, not a JSON string scalar.
• Add focused PGLite coverage plus a real Postgres E2E regression for the postgres.js direct binding path.
• Narrow the JSONB pattern guard to this checkpoint regression and fix the admin build check so local verify does not hang behind the Bun wrapper.

Upstream comparison

Compared against #2288: same root cause, but this branch binds a wrapper object and extracts completed_keys server-side via ($3::jsonb)->'completed_keys'. That keeps the existing direct-pool write path while avoiding JSON.stringify(sorted) double-encoding.

The first PR I opened from this machine (#2292) was closed because it was based on the fork master lineage and included unrelated history. This is the rebuilt upstream-based PR.

Proof receipts

• bun run verify -> [verify-parallel] elapsed=95s | pass=30 fail=0 | all checks green
• bun test test/op-checkpoint.test.ts -> 31 pass, 0 fail
• scripts/check-jsonb-pattern.sh -> all three JSONB/max_stalled checks OK
• Fresh pgvector/pgvector:pg16 Postgres E2E: DATABASE_URL=postgresql://postgres:postgres@localhost:5435/gbrain_test bun test test/e2e/op-checkpoint-jsonb-postgres.test.ts -> 1 pass, 0 fail
• Direct Postgres readback: SELECT jsonb_typeof(completed_keys), completed_keys->>0, jsonb_array_length(completed_keys) ... -> array|chunk-a|2