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>

enodia_sentinel/config.py

@@ -133,6 +133,7 @@ class Config:
     posture_sudoers: str = "/etc/sudoers"
     posture_sudoers_dir: str = "/etc/sudoers.d"
     posture_path: tuple[str, ...] = ()       # PATH dirs to audit ('' = safe default set)
+    posture_systemd: bool = True             # audit enabled systemd units (shells out to systemctl)
 
     # eBPF on-ramp
     capture_execve_bpftrace: bool = False