docs: add Attested Confidential Inference draft

[h4x3rotab]

Summary

Adds a draft Attested Confidential Inference (ACI) spec under docs/specs/ and links it from the README reference section.

ACI defines a vendor-neutral interface for attested private AI inference services: workload identity and keyset binding, service-scoped attestation reports, client-facing E2EE v2, signed inference receipts, and a dstack vLLM proxy compatibility appendix.

Reference implementation: https://github.com/Dstack-TEE/private-ai-gateway

Validation

• git diff --check
• stale local planning references scanned in the added spec

Gateway validation before publishing the reference repo:

• cargo fmt --all -- --check
• cargo test --all-targets
• live provider checks previously passed for dstack KMS/quote, Tinfoil, NEAR AI, and Chutes paths