feat: Add PR template to guide base branch selection

Adds a pull request template that reminds developers to target fork-main instead of main. The template includes:
- Clear warning about base branch selection
- Explanation of branch purposes (fork-main vs main)
- Standard PR sections (Why, How, Test Plan)
- Link to FORK.md documentation

This helps prevent accidentally creating PRs against the locked main branch which is reserved for automated upstream syncing.

Author: americodls

.github/pull_request_template.md

@@ -0,0 +1,26 @@
+## ⚠️ Important: Check Your Base Branch
+
+**This PR should target `fork-main`, not `main`.**
+
+- ✅ **`fork-main`** ← Active development branch (use this!)
+- ❌ **`main`** ← Auto-synced upstream mirror (locked, do not use)
+
+If you accidentally selected `main` as the base branch, change it using the dropdown above before submitting.
+
+---
+
+## Why?
+
+[Brief description of the problem or opportunity this PR addresses]
+
+## How?
+
+[High-level approach and key implementation decisions]
+
+## Test Plan
+
+[How to verify the changes work as expected]
+
+---
+
+📚 **New to this fork?** Read [FORK.md](../FORK.md) to understand our branch strategy.