Skills configured through `skills.external_dirs` in config.yaml were
visible via `hermes skills list`, `get_skill_commands()`, and the
agent's `/skill-name` dispatch, but silently excluded from the
Telegram and Discord slash-command menus. The filter in
`_collect_gateway_skill_entries` only accepted skills whose
`skill_md_path` started with `SKILLS_DIR`, so anything under an
external directory fell through.
Widen the accepted-prefix set to include all configured external
dirs alongside the local skills dir. Every prefix is now
slash-terminated so `/my-skills` cannot also admit
`/my-skills-extra`. Also guard against empty `skill_md_path`
values so they can't accidentally match.
Fixes#8110
Salvages #8790 by luyao618.
Co-authored-by: Yao <34041715+luyao618@users.noreply.github.com>