Fix unused analysis, JSON envelope, config parsing, locks, and recipe coverage

[Widthdom]

Summary

• Fix cdidx unused C# partial-class private member handling and unused kind output for constants/fields.
• Bound --json-envelope raw JSON aggregation by item and node budgets.
• Replace exception-based config integer parsing with explicit TryParse diagnostics.
• Share exclusive file lock helpers across CLI, MCP, and suggestion store lock paths.
• Add built-in search audit recipe snapshot and unknown-recipe diagnostics coverage.

Scope Notes

• No requested issue was excluded; all five listed issues had no prior work-start comment when checked.
• No unrelated follow-up issue was filed.
• AGENT.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --filter "FullyQualifiedName~CdidxConfigFileTests|FullyQualifiedName~JsonEnvelopeWrapperTests|FullyQualifiedName~RunSearch_BuiltInRecipeSnapshotCoversNamesScopesAndQueries_Issue3692|FullyQualifiedName~RunSearch_UnknownRecipeErrorListsBuiltInNames_Issue3692|FullyQualifiedName~ExclusiveFileLockTests|FullyQualifiedName~RunUnused_CSharpPartialClassReferencesDoNotReportPrivateHandlers_Issue3673|FullyQualifiedName~RunUnused_CSharpFieldsAndConstantsUseSpecificKinds_Issue3673" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Fixes #3673
Fixes #3687
Fixes #3692
Fixes #3697
Fixes #3779