Add SandboxExtension base class, extension SDK docs, vm-packer attach mode for build-machines VMs, profile.vm-haskell-build, SSH port support, tests, and migration docs.
CoulombCore (92.205.130.254) authenticates as tegwick via id_ops in ssh config, not root. Omit SANDBOXER_SSH_USER to let OpenSSH apply config; set SANDBOXER_SSH_USER only to override.