feat(ci): add coverage regression detection

[Copilot]

Adds automated detection to prevent test coverage from decreasing on PRs.

Changes

• New script scripts/ci/compare-coverage.ts

  • Compares coverage-summary.json between PR and base branch
  • Reports overall and per-file coverage deltas
  • Exits non-zero on any regression (lines, statements, functions, or branches)

• Updated .github/workflows/test-coverage.yml

  • For PRs: runs coverage on both branches, compares, and fails on regression
  • Posts detailed comparison report to PR comment with per-file breakdown
  • For main pushes: unchanged behavior (report only)

Example PR Comment Output

## ✅ Coverage Check Passed

### Overall Coverage

| Metric | Base | PR | Delta |
|--------|------|-----|-------|
| Lines | 77.00% | 78.50% | 📈 +1.50% |
| Statements | 77.33% | 78.80% | 📈 +1.47% |

When regression is detected, the workflow blocks the PR with a clear message indicating which metrics decreased.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Medium Priority] Add coverage regression detection</issue_title>
<issue_description>## Context
From CI/CD Pipeline Gap Assessment (Discussion #227)

Description

Implement automated detection to prevent test coverage from decreasing on PRs. This ensures code quality doesn't regress over time.

Acceptance Criteria

• Implement coverage threshold checking in CI
• Block PRs that decrease overall coverage percentage
• Provide clear feedback on coverage changes in PR comments
• Configure appropriate coverage thresholds (e.g., maintain 80%+)
• Consider per-file coverage tracking

Estimated Effort

Approximately 4 hours

Related

• Discussion CI/CD Pipeline and Integration Tests - Gap Assessment #227</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Medium Priority] Add coverage regression detection #234

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[github-actions]

✅ Coverage Check Passed

Overall Coverage

Metric	Base	PR	Delta
Lines	77.19%	77.19%	➡️ +0.00%
Statements	77.27%	77.27%	➡️ +0.00%
Functions	77.17%	77.17%	➡️ +0.00%
Branches	69.76%	69.76%	➡️ +0.00%

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Coverage comparison generated by scripts/ci/compare-coverage.ts