Add systemd unit posture checks

Close the remaining v0.8 posture bullet: flag enabled systemd units with
group/world-writable unit files (systemd_unit_writable, sid 100050) and
units whose ExecStart binary runs from a writable/unsafe path such as
/tmp, /dev/shm, /var/tmp, /home, or /run/user (systemd_exec_unsafe_path,
sid 100051).

- posture.systemd_findings/parse_systemctl_show/read_systemd_units, wired
  into posture.run() behind the new posture_systemd config knob.
- read_systemd_units shells out to systemctl with timeouts and fails closed
  on non-systemd hosts; the dashboard /api/posture surfaces findings for free.
- Unit tests for the parser and evaluator; TestRunIntegration patches the
  new reader so run() never touches live systemctl.
- Docs: COMMAND_REFERENCE posture list, ROADMAP v0.8 bullet, RUNBOOKS
  Runbook 2 triggers, config TOML knob.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -31,9 +31,9 @@ work that is bigger than single alerts.
 - ✅ Add `enodia-sentinel incident list/show/export`.
 - ✅ Add host posture checks (`enodia-sentinel posture check`):
   SSH root/password/empty-password login, passwordless and writable sudoers,
-  world-writable PATH components, loose sensitive-file permissions, and disabled
-  package signatures. Still to come: unexpected enabled services and unsafe
-  systemd unit settings.
+  world-writable PATH components, loose sensitive-file permissions, disabled
+  package signatures, and enabled systemd units with writable unit files or
+  `ExecStart` binaries in writable/unsafe paths.
 - ✅ Add a machine-readable `status --json` command for automation.
 - ✅ Update the red-team harness so every current detection has a named drill and
   expected `sid` (`sentinel-redteam --list`), with a PASS/MISS verification pass.