feat: multi-architecture builds (x86_64 + arm64)

- build-rpm.sh: --arch flag for targeting x86_64 or arm64, --all for both
  Uses bun cross-compile with --target=bun-linux-x64/arm64
- build-bastion.sh: --arch flag for Docker platform targeting
- release.sh: builds both architectures by default
- CI: builds + publishes RPM/DEB for both architectures

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bastion/scripts/release.sh

@@ -13,8 +13,8 @@ fi
 echo "=== lab-bastion release ==="
 echo ""
 
-# 1. Build binaries & packages
-bash scripts/build-rpm.sh
+# 1. Build binaries & packages (both architectures)
+bash scripts/build-rpm.sh --all
 
 echo ""