Add configurable agent spawn depth (#12251)

Summary
- expose `agents.max_depth` in config schema and toml parsing, with
defaults and validation
- thread-spawn depth guards and multi-agent handler now respect the
configured limit instead of a hardcoded value
- ensure documentation and helpers account for agent depth limits

codex-rs/core/src/codex.rs

@@ -11,7 +11,6 @@ use crate::CodexAuth;
 use crate::SandboxState;
 use crate::agent::AgentControl;
 use crate::agent::AgentStatus;
-use crate::agent::MAX_THREAD_SPAWN_DEPTH;
 use crate::agent::agent_status_from_event;
 use crate::analytics_client::AnalyticsEventsClient;
 use crate::analytics_client::AppInvocation;
@@ -315,7 +314,7 @@ impl Codex {
         }
 
         if let SessionSource::SubAgent(SubAgentSource::ThreadSpawn { depth, .. }) = session_source
-            && depth >= MAX_THREAD_SPAWN_DEPTH
+            && depth >= config.agent_max_depth
         {
             config.features.disable(Feature::Collab);
         }