Deprecate find file_count JSON alias

[Widthdom]

Summary:

• Document files as the canonical cdidx find --count --json matched-file count.
• Keep file_count as a deprecated compatibility alias with matching value and next-major removal timing.
• Add a focused regression assertion and changelog fragment.

Validation:

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~QueryCommandRunnerTests.RunFind_CountOnlyJsonUsesFilesAsCanonicalCountAndKeepsDeprecatedAlias_Issue1423 -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• adversarial review: no blocking findings

Fixes #1423