Enhancement: reserved @everyone identity to mention all channel members

## Problem

There's currently no way to notify every member of a channel in one message. `buzz messages send` only supports explicit, repeatable `--mention ` for individual recipients — a plain `#channel` tag with no explicit `@Name`/`--mention` does not trigger any bot/agent replies, and there is no wildcard/broadcast mention.

## Proposal

Reserve `@everyone` (or similar, e.g. `@here`/`@channel`) as a special identity name that resolves to all current members of the channel the message is posted in, notifying each of them the same way an explicit `@Name` mention does today.

This would require:
- Reserving `@everyone` so it can never collide with an actual user/agent display name.
- Resolution logic in the CLI/relay client to expand `@everyone` into the full current member list of the target channel at send time.
- Same delivery guarantees as explicit mentions (i.e. it should actually trigger notifications/agent wake-ups for every member, not just tag them cosmetically).

## Why

Multi-agent channels regularly need to loop in every participant (e.g. broadcasting a decision or asking all channel agents to weigh in), and today that requires manually enumerating every member's display name or pubkey, which is error-prone and easy to get wrong (ambiguous/duplicate display names can silently resolve to the wrong pubkey).

Start with the `buzz messages send` entry point and its repeatable `--mention ` handling, then trace the CLI/relay client path that resolves mentions and triggers notifications or agent wake-ups. Define how the reserved identity is protected from display-name collisions and how the current channel member list is expanded at send time, with completion requiring delivery behavior equivalent to explicit mentions.

Written by the indexing model from the issue text.