Add announces subcommand to internal/cmd/mail.go that provides:
- gt mail announces: Lists all announce channels from messaging.json
- gt mail announces <channel>: Reads messages from a specific channel
The command queries beads for messages with announce_channel label and
displays them in reverse chronological order. Messages are NOT marked
as read or removed, preserving bulletin board semantics.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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