feat(supervisor): prototype Pi conversation middleware

[matthewgrossman]

DO NOT MERGE THIS

Summary

Build a narrow working prototype that lets a Pi extension send complete conversation hooks through a stable supervisor bridge to operator-run gRPC middleware. The middleware redacts sandbox, signs the sanitized conversation, and fail-closed egress verification permits only the matching OpenAI Chat Completions request.

Related Issue

No issue required: this is an explicitly requested disposable demonstration PR and is not intended for merge.

Changes

• Add typed agent-conversation middleware bindings and a supervisor-owned loopback bridge
• Add deterministic stateless attestation signing and fail-closed HTTP verification
• Add the Pi adapter, NVIDIA Inference API model configuration, Docker example, and sandbox policy
• Log original and replacement conversations as formatted JSON for the demo
• Publish the design at architecture/pi-conversation-middleware.md
• Publish the validated walkthrough at examples/pi-conversation-middleware/TESTING.md

Testing

• mise run pre-commit passes
• Focused middleware and sandbox unit tests pass
• Local Docker gateway and sandbox demo completed
• Pi returned REDACTED and egress logged engine:middleware failed:false transformed:true
• Missing-attestation egress fails closed

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated

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