Support unused bucket and confidence filters

[Widthdom]

Summary

• Add --bucket and --min-confidence to cdidx unused and apply them to list and count paths.
• Add matching MCP unused_symbols arguments: bucket and minConfidence.
• Keep filtering based on final unused bucket/confidence classification, including exact filtered count handling.

Validation

• dotnet build src/CodeIndex/CodeIndex.csproj -c Debug --no-restore -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~RunUnused
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~ToolsCall_UnusedSymbols
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net9.0 --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~ConsoleUiTests.PrintUsage
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net9.0 --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~McpToolContractTests
• dotnet tools/CodeIndex.Changelog/bin/Debug/net8.0/CodeIndex.Changelog.dll check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• GitHub Actions for 926bb435d: Changelog Fragments, CodeQL, Build and Test all succeeded.
• Codex adversarial review: no findings.

Documentation / Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/3001.fixed.md.

Follow-up

• None.

Fixes #3001