Add Contributing guide

CONTRIBUTING.md

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+# Contributing Guide
+
+Thanks for wanting to contribute!
+
+We really appreciate both small fixes and thoughtful larger changes.
+
+## Two Paths to Get Your Pull Request Accepted
+
+### Path 1: Small, Focused Changes (Fastest way to get merged)
+- Pick **one** clear thing to fix/improve
+- Touch the **smallest possible number of files**
+- Make sure the change is very targeted and easy to review
+- All automated checks pass (including Greptile comments)
+- No new lint/test failures
+
+These almost always get merged quickly when they're clean.
+
+### Path 2: Bigger or Impactful Changes
+- **First** talk about it in Discord → #dev channel  
+  → Describe what you're trying to solve  
+  → Share rough ideas / approach
+- Once there's rough agreement, build it
+- In your PR include:
+  - Before / After screenshots (or short video if UI/behavior change)
+  - Clear description of what & why
+  - Proof it works (manual testing notes)
+  - All tests passing
+  - All Greptile + other PR comments addressed
+
+PRs that follow this path are **much** more likely to be accepted, even when they're large.
+
+## General Rules (both paths)
+- Write clear commit messages
+- Keep PR title + description meaningful
+- One PR = one logical change (unless it's a small related group)
+- Run tests locally first
+- Be kind in discussions 😄
+
+Questions? Just ask in #dev — we're happy to help.
+
+Happy hacking!