Add custom extractor configuration workflows

[Widthdom]

Summary

• Add YAML-backed custom symbol pattern sidecars.
• Add per-user and per-workspace language-map overrides.
• Add the test-extractor harness for isolated symbol extractor checks.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedNameFileIndexerTests.DetectLanguage_WorkspaceLangMapYaml_AliasesExtension|FullyQualifiedNameSymbolExtractorTests.Extract_ConfiguredPatternYaml_HandlesOutOfTreeLanguage|FullyQualifiedName~ProgramRunnerTests.Run_TestExtractor_PrintsIsolatedSymbols
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Updated README.md and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/1756.added.md.
• Added changelog.d/unreleased/1757.added.md.
• Added changelog.d/unreleased/1759.added.md.

Follow-up Candidates

• None.

Fixes #1756
Fixes #1757
Fixes #1759