Add GitHub Actions CI

[hwawshy]

Summary

Adds a CI workflow (.github/workflows/ci.yml) that builds the project on commits to the default branch and PRs against it.

• check job (fail-fast): cargo fmt --check and cargo metadata --no-deps on ubuntu-latest using the preinstalled latest-stable Rust.
• build job (needs: check): matrix over PHP versions (currently 8.5; add 8.6 once stable), provisioning a ZTS PHP 8.5 with the embed SAPI via shivammathur/setup-php (phpts: zts), installing libclang-dev (bindgen), symlinking libphp8.5.so to libphp.so (build.rs links dylib=php), then cargo clippy --all-targets -- -D warnings and cargo build --release --bin ferrumphp.
• Cargo deps (registry/git/target) cached via actions/cache, keyed on hashFiles('Cargo.lock') (exact hit when the lockfile is unchanged) with restore-keys fallback.
• Concurrency group cancels superseded runs.

Additional changes

• fix formatting: cargo fmt cleanups in src/ (the cargo fmt --check job requires a formatted tree).
• add AGENTS.md: agent instructions for building/working on this repo (from a prior session; kept in this PR for review).