ci: add github actions rust formater, linter, and test execution

[satsfy]

Depends on Rust refactor.

Summary

Add a GitHub Actions CI workflow for the Rust workspace.

This PR:

• adds a CI workflow that runs on pushes to main and on pull requests
• runs cargo fmt --check
• runs cargo clippy --workspace --all-targets -- -D warnings
• runs cargo test --workspace
• installs Bitcoin Core 29.0 in CI so node-backed tests can run in GitHub Actions
• enables Rust dependency caching to speed up repeated runs

Why

Stealth now has a Rust workspace and node-backed test coverage, so it should have baseline CI to enforce formatting, linting, and correctness on every PR.

This gives the project a consistent quality gate, catches regressions earlier, and ensures the test suite runs in an environment that includes bitcoind.