Add Codex custom parent trace config

[Stephen Belanger (Qard)]

Summary

• Add Codex-specific CODEX_PARENT_SPAN_ID and CODEX_ROOT_SPAN_ID support for attaching a Codex trace to an existing Braintrust trace.
• Add matching config.json keys: parentSpanId and rootSpanId.
• Keep Codex turns and tools parented under the Codex session span while the session span attaches under the configured external parent.
• Document the new configuration and examples.

Validation

• bun test
• bun run typecheck
• bun run lint