docs(patches): note core-go-checksum lives on the fork's master, pinned via replace

patches: track ubuntu/genimg nbd-settle and chroot-chown fixes

Two-bug patch against builds.sr.ht-images ubuntu/genimg, applied
out-of-band on work.lab.local but not yet upstreamed to sr.ht-dev.

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.