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/OPERATIONS.md

@@ -51,6 +51,7 @@ enodia-sentinel fim-check
 enodia-sentinel pkgdb-check
 enodia-sentinel pkgdb-verify --sample 100
 enodia-sentinel rootcheck
+enodia-sentinel posture check
 enodia-sentinel triage
 ```
 
@@ -62,6 +63,9 @@ Expected healthy output:
 - `pkgdb-check`: package DB consistent with anchor.
 - `pkgdb-verify`: sampled files match signed cache packages.
 - `rootcheck`: no hidden processes, modules, ports, or sniffers.
+- `posture check`: no SSH/sudo/PATH/permission/signature hygiene findings, or
+  only ones you have consciously accepted (e.g. password auth on a host that
+  needs it).
 
 ## Alert Workflow