[codex] Add query trace logging

[Widthdom]

Summary

• Add opt-in --trace=stderr|file|none handling for CLI query commands.
• Emit one sanitized JSON trace line with elapsed time, exit code, error status, and result count when it can be derived from --count/NDJSON output.
• Document the query trace option and add a bilingual changelog fragment.

Documentation and changelog

• Updated README.md with the query trace contract.
• Added changelog.d/unreleased/1901.added.md.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ProgramRunnerTests|FullyQualifiedName~CliFlagSchemaTests" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Review

• In-thread adversarial review found no blocking/actionable issues.
• codex exec adversarial review attempt was blocked by the local Codex usage limit before producing review output.

Follow-up candidates

• None.

Fixes #1901