Add Kimi (Moonshot AI) provider support

- OAuth2 device authorization grant flow (RFC 8628) for authentication
- Streaming and non-streaming chat completions via OpenAI-compatible API
- Models: kimi-k2, kimi-k2-thinking, kimi-k2.5
- CLI `--kimi-login` command for device flow auth
- Token management with automatic refresh
- Thinking/reasoning effort support for thinking-enabled models

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/cmd/auth_manager.go

@@ -19,6 +19,7 @@ func newAuthManager() *sdkAuth.Manager {
 		sdkAuth.NewQwenAuthenticator(),
 		sdkAuth.NewIFlowAuthenticator(),
 		sdkAuth.NewAntigravityAuthenticator(),
+		sdkAuth.NewKimiAuthenticator(),
 	)
 	return manager
 }