Add Rust crate consistency checks

[iancleary]

Summary

• Add GitHub Actions CI that runs the repo's contributor check entrypoint on pushes and PRs.
• Add missing MIT license metadata to the mermaid crate manifest.
• Fix two test-only clippy warnings so cargo clippy --all-targets --all-features -- -D warnings passes.

Checks run

• cargo fmt --all -- --check - pass
• cargo clippy --all-targets --all-features -- -D warnings - pass
• cargo test --all-features - pass
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --all-features - pass
• cargo package --list - pass, with existing manifest metadata warnings on internal crates
• sh scripts/check-forge-binaries.sh - pass

Notes/risks

• just ci was not run locally because just is not installed in this environment; the new workflow installs just before running it.
• cargo package --workspace still fails because workspace path dependencies such as cli-core do not specify version requirements. I left that larger packaging decision out of scope for this focused PR.
• Optional tools cargo-audit, cargo-deny, cargo-machete, and cargo-semver-checks were not installed locally, so those checks were skipped.