chore: remove iFlow-related modules and dependencies

- Deleted `iflow` provider implementation, including thinking configuration (`apply.go`) and authentication modules.
- Removed iFlow-specific tests, executors, and helpers across SDK and internal components.
- Updated all references to exclude iFlow functionality.

internal/cmd/auth_manager.go

@@ -6,7 +6,7 @@ import (
 
 // newAuthManager creates a new authentication manager instance with all supported
 // authenticators and a file-based token store. It initializes authenticators for
-// Gemini, Codex, Claude, iFlow, Antigravity, and Kimi providers.
+// Gemini, Codex, Claude, Antigravity, and Kimi providers.
 //
 // Returns:
 //   - *sdkAuth.Manager: A configured authentication manager instance
@@ -16,7 +16,6 @@ func newAuthManager() *sdkAuth.Manager {
 		sdkAuth.NewGeminiAuthenticator(),
 		sdkAuth.NewCodexAuthenticator(),
 		sdkAuth.NewClaudeAuthenticator(),
-		sdkAuth.NewIFlowAuthenticator(),
 		sdkAuth.NewAntigravityAuthenticator(),
 		sdkAuth.NewKimiAuthenticator(),
 	)