Add gt mail channel subcommands for beads-native channels:
- gt mail channel [name] - list channels or show messages
- gt mail channel list - list all channels
- gt mail channel show <name> - show channel messages
- gt mail channel create <name> [--retain-count=N] [--retain-hours=N]
- gt mail channel delete <name>
Channels are pub/sub streams for broadcast messaging with retention policies.
Messages are stored with channel:<name> label and retrieved via beads queries.
Part of gt-xfqh1e.12 (channel viewing task).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
