feat(fpagent): show scanner availability and gate capture

Consume fpagent's fprint-device-status {available, device, captureAgentId}
in the /fingerprint wizard. Availability is tracked tri-state so an agent
that does not report it leaves capture working as before; the header tag
shows offline / no-scanner / ready, the capture panel warns, and capture is
disabled until a supported reader is attached.

Adds a vm-sandbox test for the wizard logic and the OpenSpec change
fpagent-device-availability, archived with the fingerprint-wizard spec.

tools/public/assets/help.html

@@ -53,7 +53,7 @@
           <li>Follow the on-screen prompts and place each finger on the attached scanner when asked.</li>
           <li>Review the captured prints and save.</li>
         </ol>
-        <p class="help-note">The scanner runs locally through the <em>fpagent</em> service; scans never leave this machine. Domain permissions are re-checked periodically and cached for a few minutes.</p>
+        <p class="help-note">The scanner runs locally through the <em>fpagent</em> service; scans never leave this machine. Domain permissions are re-checked periodically and cached for a few minutes. The wizard shows whether a supported scanner is connected and disables capture until one is.</p>
       </cds-accordion-item>
     </cds-accordion>