fix: cap GitHub Actions annotations at 50 with overflow notice

[mcarvin8]

Summary

• generateGitHubActions previously emitted one ::warning per uncovered line with no upper bound
• GitHub Actions silently truncates annotations beyond its per-step limit, giving no indication to the user that lines were dropped
• Annotations are now capped at 50. When the total exceeds the cap, a ::notice is appended stating how many additional uncovered lines were not shown
• The summary ::notice and handler data are unaffected — the full count is still reported accurately in the summary line

Test plan

• All 107 unit tests pass (npm run test:only)
• 100% branch coverage (196/196)
• New test: 60 uncovered lines → 50 ::warning lines + truncation notice citing 10 remaining
• New test: 51 uncovered lines → singular "line" in truncation notice
• New test: ≤50 uncovered lines → no truncation notice emitted

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
...c/transformers/generators/generateGitHubActions.ts	100.00% <100.00%> (ø)

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