Add CI workflow to build kernel + run health check on push

[Shreyas0047]

The repo currently has no CI pipeline. Every change needs manual testing with ./build.sh && ./test.sh --check. A GitHub Actions workflow would:

• Run make defconfig + merge custom-os.config
• Build the kernel with embedded initramfs
• Boot in QEMU and run --check to verify login prompt
• Run both direct kernel and --bios (GRUB) boot modes

This catches regressions immediately and gives the repo a green "build passing" badge.

Suggested trigger: push to main and pull_request targeting main.