Fix native DeepSeek desktop handoff

[erickkkyt]

Summary

• pin both the parent and DeepSeek catalog entries to plaintext multi-agent v1
• detect and repair DeepSeek v2 drift plus safely upgrade manager-owned agent contracts
• forbid parent-generated fallback when a child reports a missing assignment
• document an end-to-end Desktop smoke test

Verification

• Python 3.9: 47 tests passed
• Python 3.12: 47 tests passed
• py_compile, JSON validation, Skill validation, diff check
• local repair: direct DeepSeek API and native Codex child metadata verification passed