Remove unused Git LFS pointer-size constant

[Widthdom]

Summary

• Remove the unused duplicate FileIndexer.GitLfsPointerMaxBytes constant.
• Keep FileContentLoader as the sole owner and user of the unchanged Git LFS pointer-size threshold.
• Add a bilingual internal changelog fragment.

Root cause

FileIndexer retained a private duplicate after Git LFS pointer detection was owned by FileContentLoader. The duplicate had no references and could drift from the active threshold.

Impact

No runtime, CLI, or MCP behavior changes. The change removes ambiguity over which component owns the threshold.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Debug --no-restore -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore -p:UseSharedCompilation=false -m:1 --filter FullyQualifiedName~GitLfs (net8.0: 4 passed; net9.0: 4 passed)
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• Local index rebuilt and verified fresh against HEAD
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Added changelog.d/unreleased/4587.internal.md with English and Japanese entries.
• No user documentation changes were required because runtime, CLI, MCP, and indexing behavior are unchanged.

Follow-up candidates

• None.

Fixes #4587