Skills for Cloud Functions

## Problem statement

There is no dedicated skill for Cloud Functions (Node.js or Python) in the firebase/agent-skills repo. Users who want to use agent skills for cloud functions development have no documented reference.

## Proposed solution

Add a new skill directory `skills/firebase-cloud-functions-basics` (or similar) that covers:

- Deploying a function using `firebase deploy --only functions`
- Triggering functions via HTTP, Pub/Sub, or Scheduled triggers
- Setting environment variables and secrets
- Testing functions locally with `firebase emulators:start`
- Integration with the Firebase UI console

## Use case

A developer building a Firebase cloud function can use this skill to understand the correct commands and patterns for deploying, testing, and integrating with other Firebase services.

Start by examining the proposed skills/firebase-cloud-functions-basics directory and the existing skills in the repository to follow their structure. Cover deployment with firebase deploy --only functions, HTTP, Pub/Sub, and scheduled triggers, environment variables and secrets, local testing with firebase emulators:start, and Firebase UI integration; done means the new skill provides these documented references.

Written by the indexing model from the issue text.