Skill for using Automatic Base Image Updates (ABIU) with Firebase App Hosting

[falahat]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces documentation for Node.js runtimes and Automatic Base Image Updates (ABIU) in Firebase App Hosting, including a new reference guide and instructions for AI assistants. A review comment correctly points out that the apphosting:backends:create command does not support a --runtime flag, as the runtime is determined automatically by the engines.node field in package.json.

[gemini-code-assist]

The firebase-tools CLI command apphosting:backends:create does not support a --runtime flag. The Node.js runtime version is automatically determined by the engines.node field in the project's package.json during the build phase. Recommending or using this flag will result in an unknown option error.

Suggested change

	## Specifying Runtime During Backend Creation
	When creating a new backend programmatically or in non-interactive environments,
	specify the target versioned runtime using the `--runtime` flag:
	```bash
	npx -y firebase-tools@latest apphosting:backends:create --runtime nodejs22 --backend my-backend-name --primary-region us-central1
	```
	## Specifying Runtime
	The runtime version is determined by the engines.node field in your package.json. To specify or upgrade the Node.js runtime, update your package.json before deploying.