Migrate CI from CircleCI to GitHub Actions

[jonbaldie]

What to build

Replace CircleCI with a minimal GitHub Actions workflow while preserving the repository's existing verification gates and execution order. Reuse the workflow already proven on PR #18, refreshed onto current main, rather than carrying forward that stale branch.

PR #38 and the active issue work are intentionally paused until this migration lands and runs green on GitHub.

Acceptance criteria

• .circleci/config.yml is removed and .github/workflows/ci.yml runs on pushes and pull requests.
• The workflow pins Deno 2.7.6 and Node.js 22 where required.
• Test/compile runs first, Mutasaurus runs after it, and Stryker runs after Mutasaurus.
• The existing compile command, full Deno test suite, mutation commands, score enforcement, and generated-file setup remain behaviorally equivalent.
• Repository docs, agent guidance, and Serena project memory identify GitHub Actions as CI.
• The obsolete CircleCI repository webhook/integration is removed so it cannot post stale or failing statuses.
• The migration PR is green in GitHub Actions, merged, and the resulting main commit is green.
• All currently open implementation issues are natively blocked by this issue until completion.

Prior work

• PR ci: migrate CircleCI to GitHub Actions #18 demonstrated the three GitHub Actions jobs successfully but is stale and carries an obsolete CircleCI status.
• Issue Migrate CI from CircleCI to GitHub Actions #20 originally captured the handoff needed to finish that migration.

Blocked by

None — this is the prerequisite for all current implementation work.