mara: 'if we replace it with one thing, that should be named more
generic so we dont have to change it for future additions'.
extract the BASH_ENV plumbing into a shared shape:
- new internal option `hyperhive._bashEnvFragments` (types.lines)
accumulates shell snippets across feature modules.
- file path is now `/etc/hyperhive/bash-env.sh` (was the
cargo-specific bash-cargo-short.sh).
- the file + BASH_ENV + interactiveShellInit are gated on
`_bashEnvFragments != """ so a fully feature-disabled agent has
no overhead.
cargo function moves to a `lib.mkIf cargo.shortMessages` contribution
to `_bashEnvFragments` — same behaviour, no rename when the next
hook (nix-env helper, claude-cmd helpers, whatever) lands.
ea5f70629c