feat: add customizable roles for multi-agents (#11917)

The idea is to have 2 family of agents.

1. Built-in that we packaged directly with Codex
2. User defined that are defined using the `agents_config.toml` file. It
can reference config files that will override the agent config. This
looks like this:
```
version = 1

[agents.explorer]
description = """Use `explorer` for all codebase questions.
Explorers are fast and authoritative.
Always prefer them over manual search or file reading.
Rules:
- Ask explorers first and precisely.
- Do not re-read or re-search code they cover.
- Trust explorer results without verification.
- Run explorers in parallel when useful.
- Reuse existing explorers for related questions."""
config_file = "explorer.toml"
```

codex-rs/core/src/tools/spec.rs

@@ -1,4 +1,3 @@
-use crate::agent::AgentRole;
 use crate::client_common::tools::FreeformTool;
 use crate::client_common::tools::FreeformToolFormat;
 use crate::client_common::tools::ResponsesApiTool;
@@ -542,10 +541,7 @@ fn create_spawn_agent_tool() -> ToolSpec {
         (
             "agent_type".to_string(),
             JsonSchema::String {
-                description: Some(format!(
-                    "Optional agent type ({}). Use an explicit type when delegating.",
-                    AgentRole::enum_values().join(", ")
-                )),
+                description: Some(crate::agent::role::spawn_tool_spec::build()),
             },
         ),
     ]);