Fix inherited batch database provenance

[Widthdom]

Summary

• keep batch-inherited database paths, project roots, Git provenance, freshness, and JSON metadata on one repository context
• preserve child-explicit overrides, implicit-default semantics, leading-dash paths, pass-through parsing, and serial/parallel isolation
• add bilingual developer/testing documentation and a bilingual changelog fragment

Root cause

The batch parent shared only a thread-local reader. Child option, envelope, and status code could therefore re-resolve database, project, and Git metadata from the current working directory and mix two repositories in one response.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build -p:UseSharedCompilation=false --filter FullyQualifiedName~CodeIndex.Tests.QueryCommandRunnerTests.RunBatch (45/45 on net8.0 and net9.0)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build -p:UseSharedCompilation=false --filter FullyQualifiedName~Issue5083 (4/4 on net8.0 and net9.0)
• dotnet run --project tools/CodeIndex.Changelog -- check
• canonical cdidx status --check and cdidx workspace status --check
• two Codex adversarial-review rounds; all four findings were fixed and covered by regression tests

Documentation and changelog

• DEVELOPER_GUIDE.md
• TESTING_GUIDE.md
• changelog.d/unreleased/5083.fixed.md

Follow-up candidates

• None.

Fixes #5083