Add quiet mode for query stderr

[Widthdom]

Summary

• Add global --quiet, -q, --silent, and CDIDX_QUIET=1 handling that suppresses informational stderr while preserving error lines.
• Register quiet flags in CLI flag schema/completions and document the script-friendly stderr behavior.
• Add regression coverage for zero-result quiet mode, env-driven quiet mode, preserved error lines, and completion parity.

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~ProgramCliTests|FullyQualifiedName~CliFlagSchemaTests|FullyQualifiedName~ConsoleUiTests.PrintCompletions_ReportFlagSetsMatchAcrossShells"
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~ProgramCliTests.QueryQuiet"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Release/net8.0/cdidx.dll --quiet search definitely_missing_query
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• git diff --check

Documentation and Changelog

• Updated README.md.
• Added changelog.d/unreleased/1805.added.md.

Review

• Adversarial review: No blocking/actionable issues found.
• Note: an external codex exec review attempt was blocked by usage limits, so the review was completed in-thread against origin/main..HEAD.

Fixes #1805