refactor(cli): split install.ts and model-fallback.ts into focused modules

Install pipeline:
- cli-installer.ts, tui-installer.ts, tui-install-prompts.ts
- install-validators.ts

Model fallback:
- model-fallback-types.ts, fallback-chain-resolution.ts
- provider-availability.ts, provider-model-id-transform.ts

src/cli/model-fallback-types.ts

@@ -0,0 +1,29 @@
+export interface ProviderAvailability {
+	native: {
+		claude: boolean
+		openai: boolean
+		gemini: boolean
+	}
+	opencodeZen: boolean
+	copilot: boolean
+	zai: boolean
+	kimiForCoding: boolean
+	isMaxPlan: boolean
+}
+
+export interface AgentConfig {
+	model: string
+	variant?: string
+}
+
+export interface CategoryConfig {
+	model: string
+	variant?: string
+}
+
+export interface GeneratedOmoConfig {
+	$schema: string
+	agents?: Record<string, AgentConfig>
+	categories?: Record<string, CategoryConfig>
+	[key: string]: unknown
+}