Document search query semantics

[Widthdom]

Summary

• Document default multi-term search as implicit AND in USER_GUIDE and MCP search description.
• Document raw --fts operators in CLI help, USER_GUIDE, and MCP schema text.
• Document search case-sensitivity behavior by mode, including FTS5 unicode61, exact substring, and symbol exact-name matching.
• Document --no-dedup rationale and trade-offs for overlapping search chunks.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests.ToolsList" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ConsoleUiTests.PrintUsage_WithoutBanner" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test 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
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Updated USER_GUIDE.md, CLI help, and MCP search schema text.
• Added changelog fragments:
  • changelog.d/unreleased/1843.docs.md
  • changelog.d/unreleased/1844.docs.md
  • changelog.d/unreleased/1846.docs.md
  • changelog.d/unreleased/1848.docs.md

Fixes #1843
Fixes #1844
Fixes #1846
Fixes #1848