Modernize CI workflow (GitHub Actions v4/v5, test matrix)

[renecannao]

Task 3 of Phase 1

Plan: docs/superpowers/plans/2026-03-23-phase1-foundation-poc.md
Depends on: #1, #2

What

• Review scripts/ci.sh to ensure no test coverage is lost in migration
• Rewrite .github/workflows/all_tests.yml with modern Actions
• Separate jobs: lint (golangci-lint + govulncheck), test (matrix), build
• Go version matrix: 1.22 + 1.23
• OS matrix: Linux + macOS

Files

• Modify: .github/workflows/all_tests.yml

Acceptance criteria

• CI uses actions/checkout@v4 and actions/setup-go@v5
• Lint job runs golangci-lint and govulncheck
• Test job runs on Go 1.22/1.23 × Linux/macOS matrix
• Build job produces Linux binary
• No test coverage lost from old scripts/ci.sh flow