#2370 s3 datastore: setup on an empty bucket never writes a local .datastore-index.json, so the push fast path never arms

Found while reproducing swamp-club#2353 on localstack (swamp 20260922, @swamp/s3-datastore 2026.09.22.2). Seen again on fresh repos during verification.

Problem

swamp datastore setup extension against an empty bucket initialises the shard-first index remotely (trace: pullIndex.discover n=0 v2-init, writing _index/_meta.json with version 2, no partitions, commitSeq 0), but never writes a local .datastore-index.json. Every push until a full pull then logs:

[s3-sync] Index merge failed, fast path stays unarmed: NotFound ... /.datastore-index.json

so the commitSeq fast path cannot arm. Every command on a freshly set-up repo pays the slow path until a full pull happens to create the file.

Expected

v2 initialisation writes an empty local index (or the post-push index merge creates it when missing), so the fast path arms after the first successful push.

Check whether @swamp/gcs-datastore has the same initialisation path.