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>

tests/main-missing-client-and-app-id.test.js

@@ -0,0 +1,20 @@
+import { DEFAULT_ENV } from "./main.js";
+
+for (const [key, value] of Object.entries({
+  ...DEFAULT_ENV,
+  "INPUT_CLIENT-ID": "",
+  "INPUT_APP-ID": "",
+})) {
+  process.env[key] = value;
+}
+
+// Log only the error message, not the full stack trace, because the stack
+// trace contains environment-specific paths and ANSI codes that differ
+// between local and CI environments.
+const _error = console.error;
+console.error = (err) => _error(err?.message ?? err);
+
+// Verify `main` exits with an error when neither `client-id` nor `app-id` is set.
+const { default: promise } = await import("../main.js");
+await promise;
+process.exitCode = 0;