Add lightweight query output formats

[Widthdom]

Summary

• add CLI count/compact/csv/tsv output formats for query-style commands
• add MCP format support for full/count/compact search and relationship responses
• document the new formats and add changelog fragments

Fixes #1642
Fixes #1941

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedNameQueryCommandRunnerTests.ParseArgs_AcceptsLightweightOutputFormats|FullyQualifiedNameQueryCommandRunnerTests.RunSearch_FormatCompactEmitsFileLineOnly_Issue1642|FullyQualifiedNameQueryCommandRunnerTests.RunSearch_FormatCsvEmitsDelimitedRows_Issue1941|FullyQualifiedNameMcpServerTests.ToolsCall_SearchFormatCompactEmitsFileLineOnly_Issue1642|FullyQualifiedName~McpServerTests.ToolsCall_SearchFormatCountAliasesCountOnly_Issue1642"
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check

Notes

• Full solution test run was attempted but stopped after repeated environment failures from git signing passphrase prompts and cascading Interop.Sys.GetCwd() FileNotFoundException failures.
• Required codex adversarial review was attempted with codex exec, but the command failed due Codex usage limit before producing findings.