F22 (andrexibiza): `scripts/check --only heath`, `--only ,` and `--only heath,rof`
filtered the job table down to nothing and reported "0 checks, ok" / exit 0, and
`--only health,rof` silently dropped the unknown name. A typo in a developer or
automation command could claim success without running the requested check.
The whole selector set is now validated before any job is built or dispatched:
an empty selection or any unknown name is a usage error (exit 2) that lists the
valid names. A valid selector still runs its check and keeps its status.
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