docs: add release checklist

Author: SayanthRock

docs/RELEASE_CHECKLIST.md

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+# Release Checklist
+
+Use this before sharing a ZIP or APK with customers.
+
+## Required checks
+
+- Module ZIP builds from GitHub Actions
+- Phone starts normally after install
+- Remove method is tested
+- Wallpaper guidance is tested
+- Theme folder behavior is tested
+- Customer support template is ready
+- Known problems are written in release notes
+- Device model and firmware are listed
+
+## Release levels
+
+| Level | Meaning |
+|---|---|
+| Alpha | Developer testing only |
+| Beta | Trusted testers only |
+| Stable | Tested on listed device and firmware |
+
+## Stable release needs
+
+- At least one OPPO device test
+- At least one realme device test
+- At least one OnePlus device test
+- Android version confirmed
+- Remove method confirmed