fix: rename binding to createOctokit and harden option merging

- Rename context binding from getOctokit to createOctokit to avoid
  SyntaxError when users write const { getOctokit } = require(...)
  in their scripts (~10 public workflows affected)
- Strip undefined values from user options to prevent clobbering
  defaults (e.g. GHES baseUrl)
- Deep-merge retry options alongside request options
- Use nullish coalescing (??) instead of logical OR (||)
- Shallow-copy opts to prevent shared reference mutation
- Add tests: undefined stripping, retry merge, falsy value preservation,
  no mutation of defaults
- 32 tests passing, lint clean, dist rebuilt

types/async-function.d.ts

@@ -11,7 +11,7 @@ export declare type AsyncFunctionArguments = {
     core: typeof core;
     github: InstanceType<typeof GitHub>;
     octokit: InstanceType<typeof GitHub>;
-    getOctokit: typeof getOctokit;
+    createOctokit: typeof getOctokit;
     exec: typeof exec;
     glob: typeof glob;
     io: typeof io;