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/tui/oauth_tab.go

@@ -24,7 +24,6 @@ var oauthProviders = []oauthProvider{
 	{"Codex (OpenAI)", "codex-auth-url", "🟩"},
 	{"Antigravity", "antigravity-auth-url", "🟪"},
 	{"Kimi", "kimi-auth-url", "🟫"},
-	{"IFlow", "iflow-auth-url", "⬜"},
 }
 
 // oauthTabModel handles OAuth login flows.
@@ -281,8 +280,6 @@ func (m oauthTabModel) submitCallback(callbackURL string) tea.Cmd {
 					providerKey = "antigravity"
 				case "kimi-auth-url":
 					providerKey = "kimi"
-				case "iflow-auth-url":
-					providerKey = "iflow"
 				}
 				break
 			}