docs: add AI autonomy architecture

[Pigbibi]

Summary

• document the current QuantStrategyLab AI automation architecture
• define automatic, human-review, and prohibited boundaries
• outline phased improvements for feedback, health-driven decisions, and guarded autonomy

Validation

• git diff --check
• python3 document sanity check for docs/ai_autonomy_architecture.md

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🤖 Codex PR Review

✅ Merge allowed: All changes are in docs/tests — Codex review skipped.

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