Fix status freshness and hotspot rebuild guidance

[Widthdom]

Summary

• Pins status --check stays stale after changed-between refresh #2808 with regression coverage showing successful --changed-between refresh keeps status --check fresh via indexed_head_sha even when the full-scan indexed_head_commit remains older.
• Updates status hotspot_family_ready recommendation does not clear degraded state after index refresh #2917 hotspot-family degraded guidance to recommend cdidx index --rebuild when unchanged rows may need restamping.
• Adds bilingual changelog fragments for both issues and updates README/USER_GUIDE guidance.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter "FullyQualifiedNameRunStatus_Json_ReportsHotspotFamilyTrustSignals|FullyQualifiedNameRunStatus_Explain_HotspotFamilyReadyRecommendsRebuild|FullyQualifiedNameRunStatusCheck_AfterChangedBetweenRefreshAtHead_TreatsCurrentIndexedHeadShaAsFresh_2808|FullyQualifiedNameJsonOutputSnapshotTests"
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --changed-between 602cfe9 1e57df8 --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Review

• Adversarial review: No blocking/actionable issues found.

Fixes #2808
Fixes #2917