Add ctags and index archive export/import - #2746

Summary

• Add cdidx export ctags with --output and --db support.
• Add portable CodeIndex archive export/import with manifest validation and --prune-paths.
• Document the new commands and add issue-based changelog fragments.

Fixes #1649
Fixes #1726

Validation

• dotnet build
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter ProgramCliTests --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ProgramCliTests.ExportImportArchive_RestoresCodeIndexDatabase|FullyQualifiedName~ProgramCliTests.ExportArchive_RejectsSourceDatabaseAsOutput|FullyQualifiedName~ProgramCliTests.ImportArchive_RemovesStaleDestinationSidecars|FullyQualifiedName~ProgramCliTests.ExportCtags_WritesTagsFileFromIndexedSymbols"

Review

• Codex adversarial review: No blocking/actionable issues found.

Notes

• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes.
• Full dotnet test was attempted separately; its captured output emitted failure lines from existing query tests despite an exit code of 0, so this PR relies on the focused CLI validation above.