- swarm-archive.nix, lib/archive-server.nix: deploy.archive.backend and
deploy.archive.s3.* options; the s3 server and the prune read the key from the
store into rclone's environment; the prune lists and addresses repositories
through rclone instead of a local directory
- swarm-bao.nix: the archive reads swarm/services/archive/s3/*
- docs: archive.md "External S3" section, two credentials-reference rows
- module-eval: builder.nix cases for local and s3, the missing-setting
assertion; bao-grants.nix gains the s3 read grant
- vm test: second node with a fake rclone serve s3 whose key is generated at
test time
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