Fix config defaults and query DB resolution

[Widthdom]

Summary

• Resolve implicit query DB paths from nested directories, including XDG-backed workspace data dirs.
• Add configurable query defaults via env/config, status --config, and validate-config.
• Document behavior and add unreleased changelog fragments.

Validation

• dotnet test CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameDbPathResolverTests|FullyQualifiedNameQueryCommandRunnerTests|FullyQualifiedNameConsoleUiTests|FullyQualifiedNameCliFlagSchemaTests|FullyQualifiedName~CdidxConfigFileTests" -p:UseSharedCompilation=false
• Codex adversarial review: No blocking/actionable issues found.

Follow-up

• Observed unrelated indexing liveness issue: cdidx index can stall for minutes in symbol extraction during repository self-index #2680

Agent files

• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Fixes #1926
Fixes #1813
Fixes #2036
Fixes #1806