[codex] Add focused search audit recipes

[Widthdom]

Summary

• Add focused built-in search audit recipes for JSON parsing, .NET risk patterns, XML parser security, filesystem traversal, and broad token audits.
• Add source-scope defaults, exclusion diagnostics, per-query top_files / truncated metadata, and recipe pagination improvements across CLI and MCP search surfaces.
• Extend CLI/MCP tests, user guide documentation, and the unreleased changelog fragment for the new recipe behavior.

Validation

• dotnet tools/CodeIndex.Changelog/bin/Debug/net8.0/CodeIndex.Changelog.dll check passed.
• git diff --check passed before commit.
• Adversarial review found no blocking/actionable issues.
• Full dotnet build / dotnet test could not complete in the Codex sandbox because MSBuild writes to obj/, bin/, and temp command files were denied by the environment.

Documentation And Changelog

• Updated USER_GUIDE.md with CLI and MCP recipe behavior.
• Added changelog.d/unreleased/3787-3714.added.md.

Fixes #3787
Fixes #3714
Fixes #3731
Fixes #3710
Fixes #3694
Fixes #3693
Fixes #3670
Fixes #3696

Follow-Up Candidates

• Run the full Release build and test suite in a normal local or CI environment where MSBuild can write build artifacts.