feat: implement channels with membership and MCP tools
Add channel management feature with public/private channels,
membership (owner/member roles), invitations, broadcast messaging,
and full MCP tool exposure.
New files:
- internal/channels/ - types, store, service, validation, errors
- internal/mcp/channel_tools.go - 8 MCP tools for channel operations
- schema/002_channels.sql - migration for channel_invites table
MCP tools: create_channel, join_channel, leave_channel, list_channels,
invite_to_channel, kick_from_channel, send_channel_message, update_channel
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
