Route unified exec process startup through remote exec-server

Add experimental config for a websocket exec-server endpoint, select a remote backend at the unified-exec process creation seam, and wrap remote sessions behind the existing process abstraction so exec_command can stream output, write stdin, and terminate over the current exec-server protocol.

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

codex-rs/core/config.schema.json

@@ -1915,6 +1915,18 @@
       "description": "Experimental / do not use. Replaces the synthesized realtime startup context appended to websocket session instructions. An empty string disables startup context injection entirely.",
       "type": "string"
     },
+    "experimental_unified_exec_exec_server_websocket_url": {
+      "description": "Optional websocket URL for connecting to an existing `codex-exec-server`.",
+      "type": "string"
+    },
+    "experimental_unified_exec_exec_server_workspace_root": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      ],
+      "description": "Optional absolute path to the executor-visible workspace root that corresponds to the local session cwd."
+    },
     "experimental_use_freeform_apply_patch": {
       "type": "boolean"
     },