post-deployment pipeline steps

[rsgalloway]

After all targets are deployed, distman checks for "post_deploy" key.

• It runs each step, in order, similar to how pipeline steps are handled now.
• Optional: allow "env" injection for variable substitution, same as regular pipelines.
• Can be implemented exactly like "pipeline" at the top level, just deferred to the end.

Use cases:

• [Reload systemd for new unit files "script": ["systemctl daemon-reload"]
• Enable or restart services "script": ["systemctl restart apache2"]
• Run migration or setup script "script": ["bash /opt/scripts/initialize.sh"]
• Notify via webhook or Slack "script": ["curl -X POST ..."]

{
  "author": "ryan@rsg.io",
  "targets": {
    ...
  },
  "post_deploy": {
    "reload_systemd": {
      "script": ["systemctl daemon-reexec", "systemctl daemon-reload"]
    },
    "enable_timers": {
      "script": [
        "systemctl enable --now deploy@api.timer"
      ]
    }
  }
}