refactor: use CI artifacts in release workflow instead of rebuilding

- Download pre-built wheels from successful CI runs
- Avoids duplicate builds and ensures consistency
- CI artifacts are already tested across all platforms
- Faster release process (no build time)
- Updates release documentation to reflect new flow

This ensures the released packages are exactly what was tested in CI.

docs/RELEASE.md

@@ -20,6 +20,7 @@ Before releasing, ensure:
 
 **What happens:**
 - ✅ Validates version format
+- ✅ Downloads pre-built packages from CI (no rebuild needed!)
 - ✅ Updates all package versions
 - ✅ Optionally tests on TestPyPI
 - ✅ Creates tag and GitHub release