feat: support service type without repo or build

[nilshamerlinck]

Summary

Adds support for deploying simple system-binary services (e.g. mailhog) as systemd user units without any git repository or build step. Previously, configure required a repo_url and service type required a build command; update and status would crash on such instances.

Minimal config that now works:

mailhog-foodcoop12-staging-superquinquin:
  ssh_host: foodcoop12-stag02
  type: service
  exec_start: /usr/local/bin/mailhog -smtp-bind-addr mailhog-foodcoop12-staging-superquinquin:2025 -ui-bind-addr mailhog-foodcoop12-staging-superquinquin:9025

configure — when type: service and no repo_url:

• skips git clone, creates the instance directory directly (used as WorkingDirectory)
• build is now optional; runs when present, silently skipped when absent

update — when type: service and no repo_url:

• skips git pull and build entirely, goes straight to systemctl --user restart

status — when no repo_url in config:

• skips all git commands, omits Remote / Branch lines from output

Existing service instances with repo_url + build are unaffected.

Test plan

• configure on a no-repo service creates the unit and starts it
• update on a no-repo service restarts the unit without git errors
• status on a no-repo service shows Instance and Unit lines only
• Existing repo-backed service instances still configure/update/show status correctly
• All unit tests pass (28 passed)

[hailangvn]

LGTM. Thank you.