docs(nostr): document #h requirement for live reaction subscriptions …

[uwagz]

…(block#3487)

What this fixes

fan_out_scoped (crates/buzz-relay/src/subscription.rs:278-394) enforces a deliberate, symmetric scoping invariant — documented in the code itself:

Global subscriptions (channel_id = None) do NOT receive channel-scoped
events. Channel-scoped subscriptions do NOT receive global events.

The relay derives a reaction's stored channel from its #e target at ingest — client-supplied #h is ignored for channel determination (NOSTR.md:50 documents this for writing). The consequence for reading is that every reaction is a channel-scoped event, so a live subscription {"kinds":[7]} without #h is a global subscription and silently receives no reactions at all — no error, no CLOSED, just nothing. The working form is {"kinds":[7],"#h":["<channel-uuid>"]}, and it works regardless of how the reaction was signed: explicit h tags on the event are matched directly, and tagless reactions match via the stored channel fallback (crates/buzz-core/src/filter.rs:78-91 — fallback applies only when the event has no h tags; explicit tags are authoritative).

NOSTR.md already documents this exact pitfall for group-metadata events:

Note: Channel-scoped storage means live global subscriptions
({kinds:[39000]}) won't receive these via fan-out. (NOSTR.md:124-126)

…but has no equivalent note for reactions, which is the case a bot/integration author is far more likely to hit: any client that wants to observe approvals/reactions live (workflow reaction-triggers make this a first-class pattern in Buzz) will naturally try a kinds-only REQ first and conclude reactions are broken. We lost real debugging time to exactly this while building a headless integration (https://github.com/OriginTrail/buzz-dkg-integration); the behavior is by design, only the docs are missing.

What this PR changes

Docs only (NOSTR.md): a subscribe-to-reactions example in "Sending Messages", plus one note mirroring the existing 39000 note. No code changes.

How to verify

• Behavior: with the relay running, open a live REQ {"kinds":[7]} (no #h) and react to a channel message from another client → nothing is delivered; re-subscribe with {"kinds":[7],"#h":["<channel-uuid>"]} → the reaction arrives.
• Claims against code (verified at 485d03a): scoping invariant crates/buzz-relay/src/subscription.rs:386-393; channel derivation derive_reaction_channel() in
  crates/buzz-relay/src/handlers/ingest.rs; #h fallback crates/buzz-core/src/filter.rs:78-91 and its test h_tag_fallback_uses_stored_channel_id.

Duplicate search: no existing issue/PR found for reactions subscription, fan-out kinds (searched 2026-07-29). DCO signed-off.

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Summary

Related issue

Testing