Extend useNostrPublish with an optional `prev` property on the event
template. For replaceable and addressable kinds, the hook automatically
manages published_at:
- First publish (no prev): set published_at equal to created_at
- Update (prev provided): preserve published_at from the old event
- Old event lacks published_at: don't fabricate one
- Caller already set published_at in tags: leave it alone
Callers pass `prev` when they have the old event from fetchFreshEvent,
giving the hook everything it needs without extra network requests.
Updated all 11 call sites that publish replaceable or addressable events.
Documents the prev convention in AGENTS.md.
Alex Gleason
c77c098843