fix(sync): op_checkpoints completed_keys double-encode blocks all sync

[immy2good]

Problem

recordCompleted (src/core/op-checkpoint.ts) blocks all gbrain sync on Postgres. It binds JSON.stringify(keys) to a $3::jsonb parameter via executeRawDirect → conn.unsafe(sql, params) with { prepare: false }. postgres.js v3 sends the JSON string as a text param, so $3::jsonb parses it into a jsonb string scalar ("[\"k\"]"), not an array. The op_checkpoints_completed_keys_array CHECK (jsonb_typeof = 'array') then rejects every sync-target checkpoint write:

[op-checkpoint] write failed (sync-target, <fp>): new row for relation "op_checkpoints"
  violates check constraint "op_checkpoints_completed_keys_array"
[sync] checkpoint target write failed — aborting before import; nothing drained.
Sync PARTIAL: imported 0 of N file(s), reason=checkpoint_unavailable.

Result: 0 files imported, last_commit never advances; "safe to retry" but every retry hits the same. Fixes #2305.

This is the parameterized twin of the ${JSON.stringify(x)}::jsonb double-encode that scripts/check-jsonb-pattern.sh guards against — the grep guard only catches the template form, so this slipped through. The in-code comment even claimed the parameterized form was safe ("NOT the double-encode trap"); it isn't, under .unsafe() + { prepare: false }.

Minimal repro:

const sql = postgres(DATABASE_URL, { prepare: false });
await sql.unsafe("select jsonb_typeof($1::jsonb) as t", [JSON.stringify(["x"])]);
// => [{ t: "string" }]   // jsonb STRING scalar, not an array

Fix

Bind the JS string[] as a Postgres text[] and build the jsonb in SQL via to_jsonb($3::text[]) — the same native binding appendCompleted already relies on (works on both Postgres and PGLite). Drops the JSON.stringify + $3::jsonb.

Test

Adds an e2e regression test in test/e2e/postgres-jsonb.test.ts asserting recordCompleted stores a jsonb array (jsonb_typeof = 'array'). The bug only manifests on real Postgres (PGLite uses a different driver path and hides it), so the test lives in the Postgres e2e suite.

Verified RED→GREEN against a real Postgres; existing PGLite op-checkpoint unit suite passes (30/0); tsc --noEmit clean.