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/fallback-chain-resolution.ts

@@ -0,0 +1,55 @@
+import {
+	AGENT_MODEL_REQUIREMENTS,
+	type FallbackEntry,
+} from "../shared/model-requirements"
+import type { ProviderAvailability } from "./model-fallback-types"
+import { isProviderAvailable } from "./provider-availability"
+import { transformModelForProvider } from "./provider-model-id-transform"
+
+export function resolveModelFromChain(
+	fallbackChain: FallbackEntry[],
+	availability: ProviderAvailability
+): { model: string; variant?: string } | null {
+	for (const entry of fallbackChain) {
+		for (const provider of entry.providers) {
+			if (isProviderAvailable(provider, availability)) {
+				const transformedModel = transformModelForProvider(provider, entry.model)
+				return {
+					model: `${provider}/${transformedModel}`,
+					variant: entry.variant,
+				}
+			}
+		}
+	}
+	return null
+}
+
+export function getSisyphusFallbackChain(): FallbackEntry[] {
+	return AGENT_MODEL_REQUIREMENTS.sisyphus.fallbackChain
+}
+
+export function isAnyFallbackEntryAvailable(
+	fallbackChain: FallbackEntry[],
+	availability: ProviderAvailability
+): boolean {
+	return fallbackChain.some((entry) =>
+		entry.providers.some((provider) => isProviderAvailable(provider, availability))
+	)
+}
+
+export function isRequiredModelAvailable(
+	requiresModel: string,
+	fallbackChain: FallbackEntry[],
+	availability: ProviderAvailability
+): boolean {
+	const matchingEntry = fallbackChain.find((entry) => entry.model === requiresModel)
+	if (!matchingEntry) return false
+	return matchingEntry.providers.some((provider) => isProviderAvailable(provider, availability))
+}
+
+export function isRequiredProviderAvailable(
+	requiredProviders: string[],
+	availability: ProviderAvailability
+): boolean {
+	return requiredProviders.some((provider) => isProviderAvailable(provider, availability))
+}