refactor(github): replace duplicated thumb-up reaction code

[github-actions]

Closes #272

Applied changes based on Issue #272.

I’ve centralized the “eyes → thumbs-up” swap to use the existing removeEyeReaction and addThumbUpReaction helpers, and updated those helpers to also handle issuesAssigned events.

Summary of changes:

• In createPullRequest, replaced the in-lined remove/insert reaction block with calls to removeEyeReaction(octokit, repo, event) and addThumbUpReaction(octokit, repo, event).
• Expanded both removeEyeReaction and addThumbUpReaction so that they treat event.action === 'assigned' the same as opened (previously they only handled opened for issues).
• Retained the surrounding try/catch in createPullRequest to log any unexpected errors.

Let me know if you’d like to tweak the logging messages or behavior further!