feat: expose getOctokit factory in script context

Add pre-configured getOctokit factory to the script context, enabling
multi-token workflows directly inside github-script.

- getOctokit(token, opts, ...plugins) inherits action config (retry, userAgent, etc.)
- Deep-merges request and retry options, deduplicates plugins
- stripUndefined prevents baseUrl clobber on GHES
- Integration test job with dynamic repo name (fork-friendly)
- 32 tests passing (15 factory + 4 integration + 6 getOctokit + 7 existing)

types/async-function.d.ts

@@ -3,6 +3,7 @@ import * as core from '@actions/core';
 import * as exec from '@actions/exec';
 import { Context } from '@actions/github/lib/context';
 import { GitHub } from '@actions/github/lib/utils';
+import { getOctokit } from '@actions/github';
 import * as glob from '@actions/glob';
 import * as io from '@actions/io';
 export declare type AsyncFunctionArguments = {
@@ -10,6 +11,7 @@ export declare type AsyncFunctionArguments = {
     core: typeof core;
     github: InstanceType<typeof GitHub>;
     octokit: InstanceType<typeof GitHub>;
+    getOctokit: typeof getOctokit;
     exec: typeof exec;
     glob: typeof glob;
     io: typeof io;