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/provider-availability.ts

@@ -0,0 +1,30 @@
+import type { InstallConfig } from "./types"
+import type { ProviderAvailability } from "./model-fallback-types"
+
+export function toProviderAvailability(config: InstallConfig): ProviderAvailability {
+	return {
+		native: {
+			claude: config.hasClaude,
+			openai: config.hasOpenAI,
+			gemini: config.hasGemini,
+		},
+		opencodeZen: config.hasOpencodeZen,
+		copilot: config.hasCopilot,
+		zai: config.hasZaiCodingPlan,
+		kimiForCoding: config.hasKimiForCoding,
+		isMaxPlan: config.isMax20,
+	}
+}
+
+export function isProviderAvailable(provider: string, availability: ProviderAvailability): boolean {
+	const mapping: Record<string, boolean> = {
+		anthropic: availability.native.claude,
+		openai: availability.native.openai,
+		google: availability.native.gemini,
+		"github-copilot": availability.copilot,
+		opencode: availability.opencodeZen,
+		"zai-coding-plan": availability.zai,
+		"kimi-for-coding": availability.kimiForCoding,
+	}
+	return mapping[provider] ?? false
+}