Document destructive filesystem cleanup audit

[Widthdom]

Summary

• Document the production File.Delete, Directory.Delete, and File.Move audit in both English and Japanese developer-guide sections.
• Classify each destructive filesystem surface by ownership, containment or boundary policy, and cleanup or rollback diagnostics.
• Add the unreleased bilingual changelog fragment at changelog.d/unreleased/4076.docs.md.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileSystemBoundaryTests|FullyQualifiedName~AtomicFileWriterTests|FullyQualifiedName~DbCommandRunnerTests.TryDeleteTemporaryDirectory|FullyQualifiedName~ProgramRunnerTests.TryValidateUpgradeInstallerDirectoryCleanupTarget" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/4076.docs.md.

Follow-up Candidates

• None.

Fixes #4076