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.

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.