feat: add 4 new error entries (silent-failures x3, concurrency-timing x1)

[htekdev]

New Error Entries

4 new entries from Strategy 2 round 250 (Stack Overflow / GitHub Issues top-voted patterns).

Entries Added

ID	File	Category	Source
sf-225	errors/silent-failures/sf-225.yml	silent-failures	actions/runner#2205 (83 reactions)
ct-103	errors/concurrency-timing/ct-103.yml	concurrency-timing	Community pattern
sf-226	errors/silent-failures/sf-226.yml	silent-failures	GitHub Actions docs + community
sf-227	errors/silent-failures/sf-227.yml	silent-failures	GitHub Actions docs + sf-068 companion

Details

• sf-225: if: always() combined with needs.result conditions still causes job skip — use !failure() && !cancelled() instead (runner#2205, 83r)
• ct-103: github.head_ref is empty string on push events; using it alone as a concurrency key collapses all push runs to one group — use github.head_ref || github.ref_name
• sf-226: github.event.commits array is absent (not null) in pull_request events — accessing commit data silently returns empty in PR workflows
• sf-227: if: failure() on a step after a continue-on-error: true step never triggers — the failure is absorbed; use steps.<id>.outcome == 'failure' instead