fix: load user's site.standard.graph.subscription records
previously /api/my-publications loaded the user's OWN
site.standard.publication records — backwards. the canonical signal
for "publications this user cares about" is the user's
site.standard.graph.subscription records (the same collection a
standard.site reader writes when you click "subscribe" on a publication).
now the toggle list shows publications you follow, enriched with
name + url from pub-search's local publications mirror when indexed.
if a followed publication isn't in the index, we still return the
at-uri so the UI can show it.
Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>