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/config.schema.json

@@ -29,6 +29,12 @@
         "$ref": "#/definitions/AgentRoleToml"
       },
       "properties": {
+        "max_depth": {
+          "description": "Maximum nesting depth allowed for spawned agent threads. Root sessions start at depth 0.",
+          "format": "int32",
+          "minimum": 1.0,
+          "type": "integer"
+        },
         "max_threads": {
           "description": "Maximum number of agent threads that can be open concurrently. When unset, no limit is enforced.",
           "format": "uint",