Add app-server environment registry for exec-server routing

Expose app-server environment register/list RPCs, persist environment selection through thread lifecycle flows, and cover the new routing behavior with app-server tests.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/v2/EnvironmentListResponse.json

@@ -0,0 +1,35 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "EnvironmentInfo": {
+      "properties": {
+        "execServerUrl": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "id": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "id"
+      ],
+      "type": "object"
+    }
+  },
+  "properties": {
+    "data": {
+      "items": {
+        "$ref": "#/definitions/EnvironmentInfo"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "data"
+  ],
+  "title": "EnvironmentListResponse",
+  "type": "object"
+}