ac-os: fix kernel firmware path for oven builds
- Remove hardcoded /workspaces/ path from config-minimal
- Dynamically inject EXTRA_FIRMWARE paths when .ucode files exist
- Firmware loads from initramfs at runtime on oven (no built-in blobs)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>