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/thinking/apply.go

@@ -18,6 +18,7 @@ var providerAppliers = map[string]ProviderApplier{
 	"codex":       nil,
 	"iflow":       nil,
 	"antigravity": nil,
+	"kimi":        nil,
 }
 
 // GetProviderApplier returns the ProviderApplier for the given provider name.
@@ -326,6 +327,9 @@ func extractThinkingConfig(body []byte, provider string) ThinkingConfig {
 			return config
 		}
 		return extractOpenAIConfig(body)
+	case "kimi":
+		// Kimi uses OpenAI-compatible reasoning_effort format
+		return extractOpenAIConfig(body)
 	default:
 		return ThinkingConfig{}
 	}