Apply chromium policy hardening

files/kiosk/chromium-kiosk-policy.json

@@ -33,6 +33,9 @@
   "ShoppingListEnabled": false,
   "EnableMediaRouter": false,
 
+  "_comment_capability_lockdown": "The distro chromium WRAPPER hardcodes --enable-extensions --enable-user-scripts --enable-printing --enable-sync --auto-ssl-client-auth (not removable via env/conf, only by bypassing the wrapper). Neutralise the capability-broadening ones HERE — policy beats cmdline flags and survives Chromium updates. ExtensionInstallBlocklist=['*'] blocks ALL extension installs, which also kills the user-scripts surface (user scripts need an extension/dev-mode). --enable-sync is already locked by SyncDisabled above. --auto-ssl-client-auth is moot: it only auto-presents a BROWSER-store client cert, and the TPM 802.1X cert isn't one — there are none; to govern explicitly, set AutoSelectCertificateForUrls (left unset = no auto-select). See files/kiosk/kiosk-session + TODO #13.",
+  "ExtensionInstallBlocklist": ["*"],
+
   "_comment_pna": "Private Network Access: the kiosk page (insecure http boot-server origin) fetches the workstation host-info agent at http://localhost:9099 — a cross-origin PRIVATE-network request Chromium blocks by default. The agent sends Access-Control-Allow-Private-Network: true; this is the matching browser-side allowance, scoped to the kiosk ORIGIN ONLY (not '*'). MUST list the origin of kiosk_url (default http://boot.fleet.lab). The machine talking to itself; no internet egress (see HOST-API.md).",
   "InsecurePrivateNetworkRequestsAllowedForUrls": [
     "http://boot.fleet.lab",