Check the code review standards with hopinion in CI
Turns on [hopinion](https://github.com/NorfairKing/hopinion) as a flake check, so the comment, style and testing guides are enforced mechanically instead of by eye in review.
`checks.x86_64-linux.hopinion-check` runs over the same package list the weeder check already uses, so a new package is covered by both without a second place to remember.
The first commit fixes the eight findings hopinion had on the tree as it stood, so that each commit is green on its own:
- Four string literals concatenated with `<>` or `++`, replaced by `concat`/`unwords` over a list of pieces.
- Two bare `-- TODO prompt` comments in `Bevel.CLI.Env`, dropped. Nothing pointed at them, and the neighbouring `die "No server configured."` has no such comment.
- Missing `genValidSpec` for `SyncRequest` and `Username`. Both are already generated in tests, so neither had coverage that its generator only produces valid values. `Username`'s spec is a new `bevel-api-server-data-gen/test/Bevel/API/Server/Data/UsernameSpec.hs`.
`nix flake check` passes (53 checks).
Not done here: `Username` has `ToJSON`/`FromJSON` and no `jsonSpec`. hopinion does not flag it, and adding one needs a new `genvalidity-sydtest-aeson` dependency on `bevel-api-server-data-gen`, so it seemed better kept out of this change.