feat: render highlights in GitHub Actions CI summary (Story 3.12)

[maximn]

Summary

• Parses the highlights array from POST /api/v1/runs response and surfaces smart signals in the GitHub Actions job summary
• Renders all 6 highlight types with severity emojis (🔴 critical, 🟡 warning, 🔵 info): new failures, health score delta, duration regression, fixed tests, new tests, known flaky
• Sorted by severity (critical first), capped at 3 signals with overflow link to dashboard
• Fully backward compatible — highlights absent or empty renders summary as before

Test plan

• All 213 tests pass (17 new across client, summary, and index)
• Verify new_failures highlights list up to 3 test names with +N more overflow
• Verify severity ordering: critical before warning before info
• Verify 4+ highlights shows "View all highlights on dashboard →" link
• Verify empty highlights array produces no Highlights section (backward compat)
• Verify dist/index.js built and committed