Handle workspace ignore and path boundary indexing

[Widthdom]

Summary

• Add regression coverage for case-insensitive built-in skip directory names.
• Load project-root .codeindex/.cdidxignore, skip nested .git repository boundaries, and preserve declared submodule traversal.
• Store and query indexed paths with Unicode NFC while keeping update-mode filesystem IO on the original path spelling.
• Normalize full-scan purge metadata so stale cleanup remains consistent for Unicode paths and nested repository boundaries.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~FileIndexerTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~DbPathResolverTests
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• Adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Updated USER_GUIDE.md and DEVELOPER_GUIDE.md for workspace .codeindex/.cdidxignore, nested repository boundaries, and NFC path storage.
• Added changelog.d/unreleased/1832.fixed.md.
• Added changelog.d/unreleased/2022.fixed.md.

Follow-up Candidates

• cdidx index can self-interrupt during full rebuild and leave the local index empty #2642 tracks the local full-index refresh interruption observed while refreshing the dogfood index.

Fixes #2023
Fixes #2022
Fixes #1832
Fixes #1947