docs: register_agent is in subagent, not in delegate

[L0Z1K]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• There is some typo related with register_agent.

[all-hands-bot]

🟢 Good taste - Clean documentation fix correcting the import path for register_agent.

[RISK ASSESSMENT]

• [Overall PR] ⚠️ Risk Assessment: 🟢 LOW
  Documentation-only change that corrects factual errors in import paths. No code execution, no breaking changes.

VERDICT:
✅ Worth merging: Fixes incorrect import examples that would confuse SDK users.

KEY INSIGHT:
Simple, necessary correction that aligns documentation with actual SDK package structure.

[xingyaoww]

Thank you!