Add workspace manifests and active workspace commands

[Widthdom]

Summary

• Add JSON workspace manifests and cdidx workspace list/status for monorepo member topology.
• Add cdidx config show and persisted active workspace commands with query-only DB resolution.
• Harden active workspace state handling and JSON error output for new workspace/config command paths.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~WorkspaceCommandRunnerTests|FullyQualifiedName~DbPathResolverTests|FullyQualifiedName~CdidxConfigFileTests|FullyQualifiedName~PropertyBasedParserTests.IsProjectPathArg|FullyQualifiedName~ProgramRunnerTests.Run_TestExtractor"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• Codex adversarial review: No blocking/actionable issues found.

Docs And Changelog

• Updated README.md and DEVELOPER_GUIDE.md.
• Added changelog fragments:
  • changelog.d/unreleased/1708.added.md
  • changelog.d/unreleased/1709.added.md
  • changelog.d/unreleased/1710.added.md

Follow-Up Candidates

• Existing indexing stall follow-ups appear covered by index: full scan stalls extracting SqlReferenceExtractor.cs symbols #2710 and Full index can stall on current repository after branch refresh #2717; no new duplicate issue was opened.

Fixes #1708
Fixes #1709
Fixes #1710