Introduce Luna.Identity project with user identity service, pairing code
generation, and channel linking. Channels (Telegram, CLI) now verify link
status before routing messages and prompt users to pair via the web
interface. WebChannel handles /link <channel-type> <code> commands to
complete the pairing flow.
Fix inverted expiration check in IsPairingCodeExpired that caused codes
to be treated as expired immediately after creation.
Comprehensive NUnit test suite covering ChannelManager, CliChannel,
TelegramChannel, DI registration, and integration scenarios. Uses
Moq for mocking with test stubs for isolated unit testing.
darman