fix(plugins): drop invalid authentication: OFF from Codex marketplace

Codex schema only accepts `ON_INSTALL` or `ON_USE`; the field is
optional, so omit it for an unauthenticated plugin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.agents/plugins/marketplace.json

@@ -12,8 +12,7 @@
         "ref": "port-codex"
       },
       "policy": {
-        "installation": "AVAILABLE",
-        "authentication": "OFF"
+        "installation": "AVAILABLE"
       },
       "category": "Development",
       "interface": {
@@ -28,8 +27,7 @@
         "ref": "port-codex-py"
       },
       "policy": {
-        "installation": "AVAILABLE",
-        "authentication": "OFF"
+        "installation": "AVAILABLE"
       },
       "category": "Development",
       "interface": {