Fix readiness trust signals

[Widthdom]

Summary

• Add row-level hotspot-family readiness verification so stamped metadata is degraded when relevant symbols still have NULL family_key values.
• Verify folded-name row completeness for read-only fallback readers before trusting fold_ready.
• Add status --json language_readiness details and document the additive status contract.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~DbReaderTests.GetHotspotFamilySignal_CurrentStampWithPartialFamilyRowsIsDegraded|FullyQualifiedName~DbReaderTests.GetStatus_ExposesPerLanguageReadinessMap|FullyQualifiedName~DbReaderTests.DegradationReasonCodes_AllCodesHaveActionableMetadata|FullyQualifiedName~DocumentationStatusContractTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --rebuild --yes --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and Changelog

• Updated README.md, DEVELOPER_GUIDE.md, and AGENT_GUIDE.md for the additive language_readiness status contract and new degradation code.
• Added changelog fragments:
  • changelog.d/unreleased/1485.fixed.md
  • changelog.d/unreleased/1607.fixed.md
  • changelog.d/unreleased/1647.fixed.md

Review

• Manual adversarial review completed for origin/main..HEAD; no blocking/actionable issues found.
• Automated nested codex exec review was attempted but could not complete in this environment because the nested agent could not execute shell commands.

Fixes #1485
Fixes #1607
Fixes #1647