Fix dangling symlink summary label

[Widthdom]

Summary

• replace the leaked output.Skipped identifier in human full-index dangling-symlink summaries with the user-facing skipped label
• extend the existing dangling-symlink fixture to cover human output while preserving its dry-run and JSON assertions
• synchronize the English/Japanese developer and testing contracts

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -m:1 -nr:false -p:BuildInParallel=false -p:UseSharedCompilation=false --no-restore --filter FullyQualifiedName~Run_DryRunAndFullScan_ClassifyDanglingSymlinkAsWarningAndUseHumanSkippedLabel_Issue4829_Issue5139 (net8.0 and net9.0 passed)
• dotnet build CodeIndex.sln -c Release -m:1 -nr:false -p:BuildInParallel=false -p:UseSharedCompilation=false --no-restore (passed with 0 warnings and 0 errors)
• dotnet test CodeIndex.sln -c Release -m:1 -nr:false -p:BuildInParallel=false -p:UseSharedCompilation=false --no-build --no-restore (net9.0: 11,280 passed / 423 skipped; net8.0: 11,758 passed / 7 skipped with one transient unrelated Git timeout fixture failure, whose exact isolated retry passed)
• dotnet run --project tools/CodeIndex.Changelog -c Release --no-build -- check (45 fragments validated)
• root status --check and workspace manifest health checks passed after commit-index refresh

Documentation and changelog

• updated DEVELOPER_GUIDE.md and TESTING_GUIDE.md in both English and Japanese sections
• added changelog.d/unreleased/5139.fixed.md

Review

• Codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

• None.

Fixes #5139