Fix cross-db deps workspace aggregation

[Widthdom]

Summary

• Add repeatable deps --workspace-db <path> support for explicit workspace member DB aggregation.
• Add cross-database file dependency edges tagged with source_db and target_db in JSON output.
• Document the new flag and add a bilingual changelog fragment.

Validation

• dotnet build
• dotnet test --filter "FullyQualifiedName~QueryCommandRunnerTests.RunDeps_WorkspaceDbJson_AggregatesAndTagsMemberDatabaseEdges|FullyQualifiedName~CliFlagSchemaTests"
• dotnet run --project tools/CodeIndex.Changelog -- check

Documentation and Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/1704.added.md.

Review

• Adversarial review completed: No blocking/actionable issues found.

Follow-up Candidates

• None.

Fixes #1704