feat: add repository workspace as default project view

- Create git file utilities (list_tree, get_file_content, list_branches, commit_file)
- Add file browsing API endpoints (list, content, branches, update)
- Create RepoWorkspace page with sidebar + main content layout
- Add FileTree component with directory navigation
- Add FileViewer component for viewing file contents
- Update project list to link to workspace
- Add workspace CSS styles
- Update router with workspace route

Quality gates: ruff ✓, mypy ✓, typecheck ✓, build ✓

apps/web/src/pages/projects.tsx

@@ -136,8 +136,8 @@ export const ProjectsPage = () => {
                 {project.description && <p className="muted">{project.description}</p>}
               </div>
               <div className="project-actions">
-                <Link className="ghost-button" to={`/projects/${project.id}/repositories`}>
-                  Repositories
+                <Link className="ghost-button" to={`/projects/${project.id}`}>
+                  Open Workspace
                 </Link>
                 <button
                   className="ghost-button"