feat: workspace backend foundation (PR-1)
- Add workspaces table migration (2026_06_01_add_workspaces)
- Create Workspace model with repo_id, user_id, branch, path, status
- Add workspace_id nullable FK to ToolInstance
- Create GitService for clone/fetch/pull/branch_exists_remotely
- Create WorkspaceManager for create/delete/sync lifecycle
- Create workspace CRUD API with 409 handling for duplicates and instances
- Wire workspace routes into FastAPI app
- 17 tests passing (8 unit + 9 integration), 1 skipped
Quality gates: ruff clean
alex
d567225bf7