feat: zero-config auto-detection mode

[maximn]

Summary

• Make report-path optional — when omitted, auto-detect test reports in common locations (**/junit.xml, **/ctrf-report.json, **/surefire-reports/*.xml, etc.)
• When no files found, log a warning listing all scanned patterns and suggest specifying report-path
• Existing explicit report-path behavior is completely unchanged (backward compatible)

Changes

File	Change
action.yml	report-path now required: false with default: ''
src/utils/auto-detect.ts	New module — scans 7 common patterns via existing discoverReportFiles(), deduplicates with Set
src/index.ts	Conditional: empty report-path → auto-detect, otherwise existing flow
src/utils/__tests__/auto-detect.test.ts	7 unit tests (patterns, dedup, sorting, empty results)
src/__tests__/index.test.ts	5 new integration tests (auto-detect entry, mixed formats, zero-file warning, explicit bypass)
dist/index.js	Rebuilt

Test plan

• 302 tests pass locally (12 new, 0 regressions)
• Verify auto-detect triggers when report-path is omitted from workflow YAML
• Verify explicit report-path still works identically
• Verify zero-file warning message includes all scanned patterns

[github-actions]

E2E Results: https://github.com/testglance/action/actions/runs/23665386703