[Feature]: DataProvider Migrations UI

Problem / motivation

SharpLsp has no first-party, open database-schema workflow. As noted in the related comment on #157, Microsoft has not open-sourced the SQL Server project engine, so SSDT/.sqlproj compatibility is not a viable path under SharpLsp's zero-proprietary-dependency rule.

DataProvider Migrations provides the open, database-agnostic alternative. It needs a first-class editor experience instead of being only a separate CLI workflow.

Proposed solution

Bake a DataProvider Migrations experience into SharpLsp and its VS Code extension:

• Discover DataProvider YAML migration schemas in the workspace and surface them in Solution Explorer.
• Provide commands/UI to create and open schemas, validate them, preview schema-diff/generated migration operations, and run DataProviderMigrate migrate.
• Surface migration progress, diagnostics, failures, and actionable fixes in the editor.
• Support secure target/provider and connection configuration through SharpLsp's existing configuration conventions.
• Refresh the UI and diagnostics when schemas or workspace configuration change.
• Integrate the migrate and export workflows, including clear provider-capability status.

Initial providers should match DataProvider Migrations today: SQLite and PostgreSQL. SQL Server support is explicitly deferred until Nimblesite.DataProvider.Migration.SqlServer exists.

This is a DataProvider-native migration workflow, not a compatibility layer for .sqlproj, SSDT, DacFx, or DACPACs.

Language relevance

Editor/tooling feature; YAML migration schemas, with SQLite and PostgreSQL initially.

Alternatives considered

• .sqlproj/SSDT/DacFx integration: rejected because the underlying database-project engine is proprietary and supporting it independently would require a ground-up reimplementation.
• DataProviderMigrate CLI only: remains supported, but does not provide workspace discovery, live diagnostics, diff preview, or in-editor migration execution.

Start by reading SharpLsp's existing VS Code extension and configuration conventions, then review how the current DataProviderMigrate CLI workflow is exposed. Done means workspace discovery, schema commands, validation, diff and migration execution, diagnostics, configuration refresh, and SQLite/PostgreSQL capability status are integrated for the proposed editor experience.