ci: migrate CircleCI to GitHub Actions

[jonbaldie]

Summary

• replace CircleCI with a GitHub Actions workflow
• preserve the test -> Mutasaurus -> Stryker gates on Deno 2.7.6
• update CI documentation, badge, Docker ignore rules, and Serena project memory

Validation

• deno check main.ts
• deno compile ... main.ts
• 177 Deno tests passed
• Mutasaurus: 99% (70/71), all files >=80%
• Stryker: 505/526 killed, all files >=80%

The existing local .serena/project.yml edit is intentionally not included.

[jonbaldie]

Superseded by #39, which reapplies the proven GitHub Actions migration onto current main, includes the requested Serena tooling refresh, and removes the obsolete CircleCI webhook before CI runs.