docs: add SourceHut integration section to the spec.sr.ht design

Records the config-driven wiring (Recipe B: pure Go, chrome copied from
compare.sr.ht), the canonical [spec.sr.ht] keys, and the shared keys read
in place.

Documents why skipping GraphQL federation is safe: api.sr.ht federates
every .sr.ht config section with no allow-list, but updateSchema skips
services that fail to serve /query, and the refresh is SIGHUP-driven
rather than a ticker.

docs: design proposal for spec.sr.ht

Reviewable document storage for the self-hosted SourceHut instance: bots
propose, humans review and curate, bots consume the approved text.

Records the four confirmed decisions (proposal-first review gate, own bare
git repos, absorb warren's read plane, thin full-loop v1) and the projects
model that gives cross-space unified search.