refactor(reactions): clean up GitHub reactions code

[github-actions]

Closes #539

Applied changes based on Issue #539.

I’ve refactored src/github/reactions.ts to eliminate the repetitive branching logic in the three export functions and centralized the endpoint selection into a single helper. Here’s what changed:

---

1. Introduced getReactionHandlers

A new function at the top that, based on the incoming GitHubEvent, returns a uniform handler object:

• create(content: string) — invokes the correct “create reaction” endpoint
• list() — lists existing reactions on the target
• delete(reaction_id: number) — deletes a reaction by ID (with a fallback for review comments)
• logTarget — human‑readable target description for logging

This replaces the three scattered if/else blocks across add/remove functions.

---

2. Simplified addEyeReaction

Before: three separate if blocks to choose issue vs. comment vs. review‑comment.
After: one call to getReactionHandlers(...), then a single .create('eyes') plus logging.

---

3. Simplified removeEyeReaction

Before: three loops over listFor* results with inline endpoint selection.
After: one handler.list(), then a .find(…) and .delete(reaction.id) if it’s the bot’s “eyes” reaction.

---

4. Simplified addThumbUpReaction

Now just calls removeEyeReaction, reuses the same handler helper, and .create('+1').

---

These changes preserve all original behavior while improving readability, reducing duplication, and making it easy to extend or adjust reaction logic in a single place.

---

Ready for review.