Problem: The S3 backend had no `:Pending s3` entry in the COMMANDS
section, `:Pending auth` only mentioned Google, the `sync` config
field omitted `s3`, `_s3_sync_id` was missing from the data format
section, `:Pending done` was implemented but undocumented, and the
README lacked a features overview.
Solution: Add `:Pending s3` and `:Pending done` command docs, rewrite
`:Pending auth` to cover all backends and sub-actions, update config
and data format references, add `aws` CLI to requirements, and add a
Features section to `README.md`.
Barrett Ruth
e9796cbce3