Add role-specific subagent nickname overrides (#13218)

## Summary
- add `nickname_candidates` to agent role config
- use role-specific nickname pools for spawned and resumed subagents
- validate and schema-generate the new config surface

## Testing
- `just fmt`
- `just write-config-schema`
- `just fix -p codex-core`
- `cargo test -p codex-core`
- `cargo test`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/config.schema.json

@@ -20,6 +20,13 @@
         "description": {
           "description": "Human-facing role documentation used in spawn tool guidance.",
           "type": "string"
+        },
+        "nickname_candidates": {
+          "description": "Candidate nicknames for agents spawned with this role.",
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
         }
       },
       "type": "object"