Add ChannelFields struct and CRUD operations for channel beads:
- ChannelFields with name, subscribers, status, retention settings
- CreateChannelBead, GetChannelBead, GetChannelByID methods
- SubscribeToChannel, UnsubscribeFromChannel for subscriber management
- UpdateChannelRetention, UpdateChannelStatus for configuration
- ListChannelBeads, LookupChannelByName, DeleteChannelBead
- Unit tests for parsing, formatting, and round-trip serialization
Part of gt-xfqh1e convoy: Beads-native messaging
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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