Define signed agent-run provenance contract

[shiny-code-bot]

Summary

• add a standalone Ed25519 compact-JWS contract for runtime-observed agent provenance
• bind repository, commit/tree, run, model family, task, transcript digest, and runtime identity to verifier challenges
• add append-only issuance replay protection, deterministic cross-language vectors, and explicit trust-boundary documentation

Security properties

• callers cannot inject runtime observations or model family
• challenge delivery must be authenticated and verification uses the original trusted challenge
• verifier leeway is capped at 300 seconds and never extends challenge authorization
• credentials and transcript contents are not emitted

Validation

• cargo test --locked --manifest-path code-rs/Cargo.toml -p code-agent-run-provenance
• uv run --with pyjwt --with cryptography python code-rs/code-agent-run-provenance/tests/verify_vector.py
• ./build-fast.sh
• git diff --check
• independent Claude Sonnet and GPT-5.5 security reviews: MERGE

Closes #442

[shiny-code-bot]

Closing immediately: this work was mistakenly targeted at the retired cbusillo/code repository. Nothing from this PR should be merged. The branch is being retained temporarily only to preserve the reviewed contract for possible migration to the active Every Code repository.