feat: support product-scoped plugins. (#15041)

1. Added SessionSource::Custom(String) and --session-source.
  2. Enforced plugin and skill products by session_source.
  3. Applied the same filtering to curated background refresh.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -8795,6 +8795,19 @@
           ],
           "type": "string"
         },
+        {
+          "additionalProperties": false,
+          "properties": {
+            "custom": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "custom"
+          ],
+          "title": "CustomSessionSource",
+          "type": "object"
+        },
         {
           "additionalProperties": false,
           "properties": {