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/RUNBOOKS.md

@@ -94,7 +94,7 @@ detector.
 ## Runbook 2 — Persistence tampering
 
 **Triggers:** `persistence` (sid 100015), `fim_modified` on a watched persistence
-path.
+path, `systemd_unit_writable` (sid 100050), `systemd_exec_unsafe_path` (sid 100051).
 
 A change to cron, a systemd unit, an SSH `authorized_keys`, a shell rc file, or
 sudo/account config — how an attacker survives a reboot.