Add first-class CODEOWNERS indexing

[Widthdom]

Summary

• recognize CODEOWNERS only at GitHub's three case-sensitive locations relative to the enclosing Git worktree root, with a non-Git scan-root fallback
• index ordered ownership rules, valid user/team/email owners, inline comments, and persistable bounded diagnostic annotations
• expose CODEOWNERS through search, files, symbols, outline, language catalogs, full/scoped/delete indexing, and branch-switch reconciliation without claiming authorization or graph semantics

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• full Release suite at the initial implementation commit: net8.0 12,050 passed / 9 skipped; net9.0 11,565 passed / 431 skipped; 0 failed
• final review-hardened branch: dotnet build tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• final focused Release suite (Issue5198 plus the language catalog contract): 9 passed on net8.0 and 9 passed on net9.0; 0 failed
• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• dogfood index refresh plus root status --check and workspace status --check (healthy, workspace matched)
• two Codex adversarial-review rounds; all seven actionable findings were reproduced and resolved

Documentation and changelog

• updated the English and Japanese CODEOWNERS contracts in USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md
• added changelog.d/unreleased/5198.added.md

Follow-up candidates

• None.

Fixes #5198