[Feature Request] Client and CLI auto-expansion of @everyone / @channel group mentions

### Summary
Currently in Buzz, sending a message that notifies participants requires explicitly adding a `@Full Name` or `--mention ` for each individual member. Un-tagged messages in channels are posted passively to the feed without sending notifications or routing events to agents.

### Problem
When users want to notify all members in a channel (or trigger all channel-assigned agents to participate in a discussion), they must manually tag each member individually. As channel size grows or team membership changes, this becomes unscalable and easy to miss participants.

### Proposed Solution
1. **Client/CLI Auto-expansion**: Add support in `buzz-cli` (and Buzz Desktop) to parse `@everyone` or `@channel` tokens in message content, auto-resolve all current member pubkeys for the target channel via channel membership metadata, and append explicit Nostr `p` tags (`--mention`) to the event before signing and broadcasting.
2. **Channel Broadcast Event Tag (Optional Protocol Extension)**: Define a standard channel-wide mention mechanism or tag convention for channel events.

### Value
Allows seamless team-wide announcements and multi-agent discussion triggers across channels without requiring manual per-member tagging.

Start at the buzz-cli message parsing and --mention entry points, then trace how channel membership metadata and Nostr p tags are handled before signing and broadcasting. Check the corresponding Buzz Desktop message path as well. Done means @everyone and @channel resolve current channel members consistently in both clients, with the protocol extension decision documented or implemented.

Written by the indexing model from the issue text.