mkosi only mounts live-image/ as /work/src, so the postinst couldn't
reach the repo-root bin/vetting-agent.linux-amd64 — the build failed
in CI with `install: cannot stat '/work/src/bin/vetting-agent.linux-amd64'`.
The Makefile now copies the prebuilt agent into mkosi.extra/, which
mkosi merges into the image root automatically. The postinst is
reduced to creating the multi-user.target.wants symlink.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>