feat: add systemd mode for EC2-like VMs

Replace shell-based init script with Go binary that supports two modes:

## Exec Mode (existing behavior)
- Go init runs as PID 1
- Starts guest-agent in background
- Runs container entrypoint as child process
- Used for standard Docker images (nginx, python, etc.)

## Systemd Mode (new)
- Auto-detected when image CMD is /sbin/init or /lib/systemd/systemd
- Go init sets up rootfs, then chroots and execs systemd
- Systemd becomes PID 1 and manages the full system
- guest-agent runs as a systemd service (hypeman-agent.service)
- Enables EC2-like experience: ssh, systemctl, journalctl all work

## Key changes:
- lib/system/init/: New Go-based init binary with modular boot phases
- lib/images/systemd.go: IsSystemdImage() auto-detection from CMD
- lib/instances/configdisk.go: Passes INIT_MODE to guest
- lib/system/init/init.sh: Shell wrapper to mount /proc /sys /dev
  before Go runtime (Go requires these during initialization)
- integration/systemd_test.go: Full E2E test verifying:
  - systemd is PID 1
  - hypeman-agent.service is active
  - journalctl works for viewing logs

## Boot flow:
1. Kernel loads initrd with busybox + Go init + guest-agent
2. init.sh mounts /proc, /sys, /dev (Go runtime needs these)
3. init.sh execs Go init binary
4. Go init mounts overlay rootfs, configures network, copies agent
5. Based on INIT_MODE: exec mode (run entrypoint) or systemd mode (chroot + exec /sbin/init)

Author: rgarcia