Add host posture checks (roadmap v0.8: posture check)

Add `enodia-sentinel posture check` — a config-hygiene audit that finds
the conditions making a host easy to attack, complementing the live
detectors. Checks: SSH root/password/empty-password login (Include-aware,
first-wins sshd resolution), passwordless and group/world-writable
sudoers, world-writable or non-root PATH directories, loose permissions
on sensitive files (/etc/shadow, /etc/passwd, ...), and downgraded
package-signature policy (reusing pkgdb.siglevel_alert).

Findings reuse the Alert type, so they serialize through the existing
JSON/triage pipeline (`posture check --json`). Each check is a pure
evaluator over injected content/stat facts plus a thin system reader, so
the 18 new tests need no root or live /etc. Posture is command-driven and
never runs in the daemon loop, per the v0.8 exit criterion.

SIDs 100040-100047 (classtype host-posture). Sample config and docs
(COMMAND_REFERENCE, OPERATIONS, ROADMAP, SPECIFICATION) updated.

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

docs/COMMAND_REFERENCE.md

@@ -155,6 +155,31 @@ Exit code:
 - `0`: sampled files match and signature policy is not downgraded.
 - `1`: a signed-package mismatch or insecure signature setting was found.
 
+### `posture check`
+
+```bash
+enodia-sentinel posture check
+enodia-sentinel posture check --json
+```
+
+Audits host configuration hygiene — the conditions that make a host *easy* to
+attack, as opposed to an attack in progress:
+
+- SSH: root login, password authentication, and empty-password logins.
+- sudo: passwordless (`NOPASSWD`) rules, disabled authentication, and
+  group/world-writable sudoers files.
+- World-writable or non-root-owned `PATH` directories (binary-hijack vectors).
+- Loose permissions on sensitive files (`/etc/shadow`, `/etc/passwd`, ...).
+- Downgraded package-signature policy (`SigLevel`).
+
+Findings are advisory and reuse the standard alert JSON shape (`--json`). This
+command does not run inside the daemon and never blocks startup.
+
+Exit code:
+
+- `0`: no posture findings.
+- `1`: one or more findings.
+
 ## Evidence and Operator Commands
 
 ### `web`
@@ -217,7 +242,6 @@ enodia-sentinel status --json
 enodia-sentinel incident list
 enodia-sentinel incident show <incident-id>
 enodia-sentinel incident export <incident-id>
-enodia-sentinel posture check
 enodia-sentinel respond plan <incident-id>
 enodia-sentinel respond apply <plan-id>
 ```