feat(cli): resolve @everyone / @channel to all channel members

[alisqr]

What

Adds a first-class @everyone / @channel broadcast keyword to buzz messages send. Typing @everyone or @channel in a message expands to a p-tag for every current channel member except the sender, so one post notifies the whole room without hand-listing pubkeys.

Motivation: users kept asking "is there a way to tag everyone at once?" There wasn't — the only workaround was buzz channels members + a repeated --mention per pubkey (or a shell script wrapping that), which is unusable from the chat UI. This makes the natural thing work.

How

buzz-sdk (mentions.rs) — two pure, network-free helpers alongside the existing mention pipeline:

• contains_everyone_keyword(content) — detects @everyone / @channel, case-insensitive, using the same boundary rules as ordinary @mention: the @ must be at start-of-input or preceded by ASCII whitespace, and the keyword must be followed by a word boundary (whitespace / common punctuation / end-of-input). Callers pass code-stripped content, so keywords inside code spans/blocks are ignored.
• is_everyone_keyword(name) — recognizes the reserved tokens so they're dropped from ordinary name resolution.
• @here is deliberately excluded — Buzz has no presence signal, so honoring it would silently over- or under-notify.

buzz-cli (resolve_content_mentions) — when the keyword is present, expands to the live member set (minus the sender) before name resolution, so the broadcast still works even if member profiles fail to load or a channel has no display names. The reserved tokens are filtered out of name matching, and the broadcast set is de-duped against name-resolved pubkeys.

The existing MENTION_CAP (50) applies unchanged: in a channel with more than 50 members, @everyone errors with the standard "too many mentions" message rather than silently truncating. Flagged here as a known boundary worth a design call if large channels need broadcast.

Tests

7 new unit tests in buzz-sdk: basic forms, case-insensitivity, trailing word-boundary, leading-boundary rule (email-like [email protected] does not trigger), multibyte-safety (no panic), and @here exclusion.

• cargo test -p buzz-sdk: 259 passed / 0 failed
• cargo test -p buzz-cli: 317 passed / 0 failed
• cargo clippy -p buzz-sdk -p buzz-cli --all-targets: clean
• cargo fmt: applied

Notes

Originated from a request in the Buzz Welcome channel. Opened from a personal fork (alisqr/buzz) per Block's contributing-to-external-OSS path, since the author lacks direct write access to block/buzz.