feat: user groups — mentionable groups of people and agents (@ios-team)

[LordMelkor]

🤖 Filed by kray's AI agent on his behalf.

Problem

Buzz has no way to address a set of members as one unit. Two Slack-parity gaps:

1. Group mentions — in Slack you define a user group (e.g. @ios-team); mentioning its handle notifies the members. In Buzz you must @-mention each person individually.
2. Bulk channel membership — putting a whole team into a channel means looking up and adding members one by one.

Buzz's existing agent teams (kind 30176) don't cover this: they are containers of persona IDs (agent templates deployed into channels) with no pubkey membership and no humans. And because Buzz members and agents are both just pubkeys, a "user group" here can naturally contain both people and agents — something Slack can't do.

The kind registry already reserves an empty 47000–47999 "User groups" range (buzz-core/src/kind.rs), so this fills in that slot.

Proposal

Community-shared, named, mentionable groups, following the established NIP-29-style pattern (signed command events + relay side effects + relay-published addressable state snapshot — same architecture as channel membership 9000→39002). NIP-51 follow sets (kind 30000) were considered but are single-author personal lists; groups need creator+admin multi-editor semantics.

Proposed kinds

Kind	Purpose
47000	create group (handle, name, description, initial p members, default channels)
47001	edit metadata / default channels
47002	delete group
47003	add member(s)
47004	remove member(s)
39100	relay-published group state snapshot (parameterized replaceable, d = group id)

Semantics

• Mentions expand to real p tags at compose time (group members ∩ channel members, Slack parity: members not in the channel are silently skipped). Every existing notification path — #p live subs, mention feed, mobile, agent wake-up — works unchanged. A ["group", <id>, <handle>] marker tag lets clients render @handle as a mention pill.
• Bulk add to channel: resolve group members → existing per-pubkey kind-9000 put-user loop (same as desktop's multi-select invite).
• Default channels (Slack parity): when a member is added to a group, the relay auto-adds them to the group's default channels (public channels only for MVP).
• Permissions: any member can create a group; only the creator and community admins can edit/delete/manage members. Handles are unique, validated ^[a-z0-9][a-z0-9_-]{1,31}$.

Surfaces (this iteration)

• Relay: command handling, Postgres storage, snapshot publishing, default-channel auto-join.
• CLI: buzz groups create|list|show|update|delete|add-member|remove-member, buzz channels add-group, group handles in message @-mention expansion.
• Desktop: groups in the @-mention selector, mention-pill rendering, groups management UI (people + agents in one picker), groups in the channel invite flow.

Out of scope / follow-ups

• Mobile (Flutter) compose/manage/render — follow-up issue (mobile already receives group-mention notifications since they're plain p tags).
• Retroactive default-channel joins when the default-channel list is edited.
• Nested groups, group roles, private groups, workflow_sink group-mention parsing (relates to workflow_sink's @-mention parser is a third divergent implementation — workflow-emitted **@Name** emits zero p tags #2686).