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>

dist/main.cjs

@@ -23153,7 +23153,7 @@ async function pRetry(input, options = {}) {
 }
 
 // lib/main.js
-async function main(appId, privateKey, owner, repositories, permissions, core, createAppAuth2, request2, skipTokenRevoke) {
+async function main(clientId, privateKey, owner, repositories, permissions, core, createAppAuth2, request2, skipTokenRevoke) {
   let parsedOwner = "";
   let parsedRepositoryNames = [];
   if (!owner && repositories.length === 0) {
@@ -23188,7 +23188,7 @@ async function main(appId, privateKey, owner, repositories, permissions, core, c
     );
   }
   const auth5 = createAppAuth2({
-    appId,
+    appId: clientId,
     privateKey,
     request: request2
   });
@@ -23307,14 +23307,17 @@ if (!process.env.GITHUB_REPOSITORY_OWNER) {
 }
 async function run() {
   ensureNativeProxySupport();
-  const appId = getInput("app-id");
+  const clientId = getInput("client-id") || getInput("app-id");
+  if (!clientId) {
+    throw new Error("Either 'client-id' or 'app-id' input must be set");
+  }
   const privateKey = getInput("private-key");
   const owner = getInput("owner");
   const repositories = getInput("repositories").split(/[\n,]+/).map((s) => s.trim()).filter((x) => x !== "");
   const skipTokenRevoke = getBooleanInput("skip-token-revoke");
   const permissions = getPermissionsFromInputs(process.env);
   return main(
-    appId,
+    clientId,
     privateKey,
     owner,
     repositories,