Fix qualified graph queries and search JSON symbol metadata

[Widthdom]

Summary

• Add qualified member query support across definitions, references, callers, and callees for issue Support qualified member queries for graph commands #2819.
• Add optional enclosing symbol metadata to search JSON and anchor it to the actual match line for issue Search JSON should include enclosing symbol metadata for each hit #2838.
• Update docs, JSON snapshot, and changelog fragments.

Validation

• git diff --check origin/main..HEAD
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net8.0 -p:UseSharedCompilation=false --filter "FullyQualifiedName~DbReaderTests.GraphReaders|FullyQualifiedName~DbReaderTests.SearchReferences_|FullyQualifiedName~DbReaderTests.GetDefinitions_ExactMatchesNameEquality|FullyQualifiedName~DbReaderTests.Search_ReturnsEnclosingSymbolMetadata_Issue2838|FullyQualifiedName~DbReaderTests.Search_ReturnsEnclosingSymbolForActualMatchLine_Issue2838|FullyQualifiedName~SearchSnippetFormatterTests|FullyQualifiedName~JsonOutputSnapshotTests.RunSearch_JsonOutput_MatchesGolden"
• 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.

Changelog

• changelog.d/unreleased/2819.added.md
• changelog.d/unreleased/2838.added.md

Notes

• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Fixes #2819
Fixes #2838