Fix ancestor ignore chain auditing

[Widthdom]

Summary

• Rebuild ancestor ignore directory resolution by walking actual parent directories from the ignore-rule root to the nested project root.
• Fail closed with scan errors when an ancestor ignore directory cannot be read, and expose the resolved ancestor list through ScanFilesResult.
• Document the nested ignore-rule resolution/audit behavior.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --filter "FullyQualifiedName~FileIndexerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• git diff --check

Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build was started, but it remained running without completion and was stopped after the relevant targeted tests had passed.

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/2024.fixed.md.

Review

• Adversarial review of origin/main..HEAD: No blocking/actionable issues found.
• A separate codex exec review attempt failed because the Codex CLI reported a usage limit.

Fixes #2024