feat: prompt enhancement endpoint (Bolt-style)

[praxstack]

Summary

Backend-only: hidden 'enhance' agent + POST /enhance API endpoint that rewrites user prompts to be clearer and more effective.

Changes (3 files, +140 lines)

• agent.ts — New hidden 'enhance' agent (temp 0.7, deny-all permissions)
• enhance.txt — System prompt with rules and examples for prompt rewriting
• experimental.ts — POST /enhance route with model fallback chain and think-tag stripping

Notes

• Graceful degradation: returns original text on any failure
• No frontend wiring yet (future branch)
• Uses existing Agent/Provider/LLM infrastructure

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