docs: document token routing benchmark

[Fission21]

Summary

• document the controlled 27-result Codex/DeepSeek token-routing benchmark in English and Chinese
• clarify that delegation is for independent implementation value, not token savings
• tighten the bundled Skill so direct execution is preferred for token/latency-sensitive and tiny tasks
• record medians, failed-attempt handling, interpretation, and limitations

Validation

• npm test (9 passed, 1 provider-backed test skipped as designed)
• git diff --cached --check