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.