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.

fix: build-index.sh — accept .git gitlinks, not just directories

After `git submodule absorbgitdirs`, each subproject's worktree contains
a .git *file* pointing into .git/modules/<name>/, not a .git directory.
The old `find -type d` test silently skipped every submodule, leaving
INDEX.md empty of repo content. Widen the type filter to both files and
directories so submodule-backed clones are detected too.

docs: document the new hut submodule

Updates CLAUDE.md (workspace layout) and the sourcehut-refresh skill to
explain that hut lives at ~xenrox, won't be discovered by the upstream
scrape, and must be bumped manually. INDEX.md timestamp bump.

init: superproject for SourceHut documentation mirror

29 upstream SourceHut repos pinned as submodules to their latest tags,
plus Claude tooling (skills, build-index script, INDEX inventory) and
phoebe-lab production patches that are not yet upstream.

Read CLAUDE.md and .claude/INDEX.md before exploring.