ci: migrate to GitHub Actions

[jonbaldie]

Closes #20.

What changed

• Replaced .circleci/config.yml with a GitHub Actions workflow for pushes and pull requests.
• Preserved the serial Test → Mutasaurus → Stryker gates and their existing commands.
• Pinned Deno 2.7.6 in every Deno job and Node.js 22 for Stryker.
• Updated the README badge, agent guidance, Docker build context, and Serena project memory.
• Included the requested Serena language_servers schema refresh.
• Removed the obsolete CircleCI repository webhook (344081286) after verifying its target was https://circleci.com/hooks/github.

Why

All active implementation work is now natively blocked by #20 so GitHub Actions becomes the verified CI foundation before those issues continue. This refreshes the workflow already proven on PR #18 onto current main without carrying that stale branch forward.

Validation

• Workflow YAML parsed and deno fmt --check .github/workflows/ci.yml passed.
• Exact compile command passed with Deno 2.7.6.
• Full Deno suite passed: 177 tests, including real socket startup/shutdown E2E tests.
• Mutasaurus passed at 99% (70/71), with every production file at or above 80%.
• Two-axis code review found no standards violations or spec gaps.
• GitHub Actions will exercise all three jobs, including Stryker, on this PR.