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-model-id-transform.ts

@@ -0,0 +1,12 @@
+export function transformModelForProvider(provider: string, model: string): string {
+	if (provider === "github-copilot") {
+		return model
+			.replace("claude-opus-4-6", "claude-opus-4.6")
+			.replace("claude-sonnet-4-5", "claude-sonnet-4.5")
+			.replace("claude-haiku-4-5", "claude-haiku-4.5")
+			.replace("claude-sonnet-4", "claude-sonnet-4")
+			.replace("gemini-3-pro", "gemini-3-pro-preview")
+			.replace("gemini-3-flash", "gemini-3-flash-preview")
+	}
+	return model
+}