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/tests/common/rollout.rs

@@ -79,6 +79,7 @@ pub fn create_fake_rollout_with_source(
         originator: "codex".to_string(),
         cli_version: "0.0.0".to_string(),
         source,
+        agent_path: None,
         agent_nickname: None,
         agent_role: None,
         model_provider: model_provider.map(str::to_string),
@@ -161,6 +162,7 @@ pub fn create_fake_rollout_with_text_elements(
         originator: "codex".to_string(),
         cli_version: "0.0.0".to_string(),
         source: SessionSource::Cli,
+        agent_path: None,
         agent_nickname: None,
         agent_role: None,
         model_provider: model_provider.map(str::to_string),