Add GitHub Actions CI workflow for build, test, and clippy across all three contracts

[abayomicornelius]

There is no .github/workflows directory and no CI configuration of any kind in the repository, despite commit history and the README's "Getting Started"/"Testing" sections referencing make build, make test, and scripts/build.sh/scripts/test.sh — none of which exist in the tree either.

Without CI, regressions in fee_collector, stellar_send, or token_bridge (all of which have real, non-trivial test suites) can merge silently. Add a workflow that runs cargo test -p <each contract>, cargo clippy, and a wasm32-unknown-unknown build for all three packages on every PR.