fix(gain): honour suppress_hook_warning, and pin the env/config composition

`rtk gain` printed its own missing-hook line from a bare `status()` match,
outside `maybe_warn`, so neither `hooks.suppress_hook_warning` nor
`RTK_SUPPRESS_HOOK_WARNING` reached it — on the report the no-hook audience
reads most. Gate that arm on the same helper; the outdated-hook prompt stays
visible, as it does everywhere else.

The new unit tests exercise the parser alone, so folding env and config
together with `||` — which drops the falsy force-off — left the whole suite
green. Drive the composition through the binary instead, and give the env
table the `=0` row that overrides the config flag.

The suite pins `XDG_CONFIG_HOME`/`XDG_DATA_HOME` to the temporary home and
seeds the config in both the XDG and the macOS location, so the loader cannot
resolve past it: `dirs::config_dir` is `~/.config` on Linux but
`~/Library/Application Support` on macOS, and a runner that exports either
`XDG_*` reached the developer's own config before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>