Bootstrap ProofPath protocol foundation

[safal207]

Summary

This PR bootstraps ProofPath as a protocol-first OSS project.

Added:

• Expanded README with project positioning and architecture
• Draft ProofPath HTTP Profile v0.1
• Initial threat model
• Roadmap
• Contributing guide
• Design note: why HTTPS is not enough for AI agents
• Initial conformance fixtures for valid and invalid requests

Why

ProofPath should start as a clear protocol and security concept before implementation. This PR gives contributors enough structure to understand the goal and pick up concrete tasks.

Next steps

• Implement minimal Rust verifier
• Expand conformance fixtures
• Build Axum gateway MVP
• Add signature profile and replay protection

Philosophy

HTTPS proves the channel. ProofPath proves the action.

[safal207]

Closing as a stale duplicate. The protocol foundation work has already landed through the later merged mainline changes, and keeping this PR open creates noise for contributors.