ci(coverage): add vitest v8 coverage with 70% line threshold

[snipcodeit]

Summary

• Configure vitest coverage with v8 provider — generates text, HTML, and lcov reports in coverage/
• Enforce 70% line coverage threshold that causes npm run test:coverage to exit non-zero if violated
• Add coverage/ to .gitignore and coverage badge to README
• Include optional CI coverage upload step with continue-on-error: true for gradual ramp-up

Closes #257

Milestone Context

• Milestone: v6 — Local Testing & Simulation Framework
• Phase: Phase 50: CI Integration & Coverage (issue 4 of 4 in milestone)
• Dependencies: Blocked by Add GitHub Actions test workflow running on every PR against Node 18 and 20 #256 → PR ci: add test.yml workflow for Node 18 and 20 matrix #267 (open — test.yml base)

Changes

vitest.config.js (new)

• Coverage provider: v8
• Reporters: text, html, lcov
• Output: ./coverage/
• Threshold: lines: 70
• Excludes: dist/, test files, config files, commands/, templates/, wiki/, docs/

package.json

• Added devDependencies: vitest@^4.0.18, @vitest/coverage-v8@^4.0.18
• Added script: "test:coverage": "vitest run --coverage"

.gitignore

• Added coverage/ to exclude generated reports from git

README.md

• Added coverage badge: [![Coverage](https://img.shields.io/badge/coverage-70%25-brightgreen)](./coverage/lcov-report/index.html)

.github/workflows/test.yml (new)

• Node 18/20 matrix test run
• Optional coverage run on node-20 (continue-on-error: true)
• Coverage artifact upload via actions/upload-artifact@v4

Note: This PR includes test.yml alongside PR #267. If #267 merges first, a minor merge conflict on test.yml will need resolution — add the coverage steps to the existing file.

Test Plan

• npm ci && npm run test:coverage completes without error
• coverage/ directory is created with lcov-report/, lcov.info, and text output
• git check-ignore -v coverage/ confirms coverage/ is gitignored
• README.md shows the coverage badge
• Modify a lib file to drop coverage below 70% — verify npm run test:coverage exits non-zero
• CI workflow runs on push/PR and uploads coverage artifact on node-20

Cross-References

• Closes Add vitest coverage reporting with 70 percent line coverage threshold #257
• Depends on Add GitHub Actions test workflow running on every PR against Node 18 and 20 #256 (PR ci: add test.yml workflow for Node 18 and 20 matrix #267)
• Part of v6 milestone: Local Testing & Simulation Framework