feat: add client-id input and deprecate app-id (#353)

GitHub now recommends using a GitHub App's Client ID for authentication.
This PR adds a first-class `client-id` input, keeps `app-id` available
for compatibility, and makes the migration path explicit in both runtime
behavior and documentation.

### Action inputs

- Adds a new `client-id` input
- Removes `required` from `app-id`
- Marks `app-id` as deprecated in `action.yml`

### Runtime behavior

- Updates input parsing to prefer `client-id`
- Falls back to `app-id` for existing workflows
- Adds a clear error when neither `client-id` nor `app-id` is provided

### Docs

- Updates the README to recommend `client-id`
- Switches usage examples to `client-id`
- Documents that `app-id` is deprecated and that `client-id` takes
precedence if both are set

### Regression coverage

- Adds a focused test proving a client-ID-shaped value works through the
new `client-id` input
- Adds coverage for the missing-ID validation path
- Updates snapshots to lock in the new metadata and runtime behavior

### Resulting usage

Users can migrate to the new input name directly:

```yaml
- uses: actions/create-github-app-token@v3
  with:
    client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
    private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: parkerbxyz <17183625+parkerbxyz@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

lib/main.js

@@ -2,7 +2,7 @@ import pRetry from "p-retry";
 // @ts-check
 
 /**
- * @param {string} appId
+ * @param {string} clientId
  * @param {string} privateKey
  * @param {string} owner
  * @param {string[]} repositories
@@ -13,7 +13,7 @@ import pRetry from "p-retry";
  * @param {boolean} skipTokenRevoke
  */
 export async function main(
-  appId,
+  clientId,
   privateKey,
   owner,
   repositories,
@@ -70,7 +70,7 @@ export async function main(
   }
 
   const auth = createAppAuth({
-    appId,
+    appId: clientId,
     privateKey,
     request,
   });