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.
Set up .NET solution structure with Luna.sln and Luna.slnx defining
all project references and build configurations for the multi-project
Luna AI assistant architecture.
darman