Make sourcehut-custom-service a global skill installed via Justfile

Move the skill from project-local .claude/skills to the skills/ backup dir
(matching sourcehut-ci) and generalize the Justfile install target to a skill
list, installing both into ~/.claude/skills. Replace absolute mirror paths with
repo::<path>::<symbol> references resolved against a single __SRHT_MIRROR__
placeholder that the install target substitutes with the repo's absolute path;
switch citations from drift-prone line numbers to stable symbol names and fix
a stale core-go/auth/internal.go reference to core-go/auth/middleware.go.

Add sourcehut-custom-service skill

Document how to build a custom service that integrates into a self-hosted
SourceHut without modifying upstream source: config-driven nav/service-switcher,
the shared sr.ht.unified-login.v1 cookie, theme reuse, GraphQL federation via
api.sr.ht, and webhooks. Includes Python/Go/other-language recipes, a full
configuration field reference, and guidance on reusing SourceHut's config key
names and accessor helpers.