feat: change multi-agent to use path-like system instead of uuids (#15313)

This PR add an URI-based system to reference agents within a tree. This
comes from a sync between research and engineering.

The main agent (the one manually spawned by a user) is always called
`/root`. Any sub-agent spawned by it will be `/root/agent_1` for example
where `agent_1` is chosen by the model.

Any agent can contact any agents using the path.

Paths can be used either in absolute or relative to the calling agents

Resume is not supported for now on this new path

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

@@ -5,6 +5,9 @@
       "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
       "type": "string"
     },
+    "AgentPath": {
+      "type": "string"
+    },
     "ApprovalsReviewer": {
       "description": "Configures who approval requests are routed to for review. Examples include sandbox escapes, blocked network access, MCP approval prompts, and ARC escalations. Defaults to `user`. `guardian_subagent` uses a carefully prompted subagent to gather relevant context and apply a risk-based decision framework before approving or denying the request.",
       "enum": [
@@ -900,6 +903,17 @@
                     "null"
                   ]
                 },
+                "agent_path": {
+                  "anyOf": [
+                    {
+                      "$ref": "#/definitions/AgentPath"
+                    },
+                    {
+                      "type": "null"
+                    }
+                  ],
+                  "default": null
+                },
                 "agent_role": {
                   "default": null,
                   "type": [