Add a native workspace directory picker to onboarding

Onboarding only accepted a typed workspace path, which is error-prone and
diverged from the original wizard. Add a pickDirectory IPC backed by Electron's
open-directory dialog and a Choose button beside the path field, so the founder
can select a real folder for agent artifacts.

AgentCompany-New/src/shared/desktop-api.ts

@@ -97,6 +97,7 @@ export interface DesktopApi {
   createScheduledPost(input: CreateScheduledPostInput): Promise<DesktopResult<AppSnapshot>>;
   setScheduledPostState(input: SetScheduledPostStateInput): Promise<DesktopResult<AppSnapshot>>;
   importFile(input: ImportFileInput): Promise<DesktopResult<AppSnapshot>>;
+  pickDirectory(): Promise<DesktopResult<string | null>>;
   setFilePinned(input: SetFilePinnedInput): Promise<DesktopResult<AppSnapshot>>;
   createMigrationReport(input: CreateMigrationReportInput): Promise<DesktopResult<AppSnapshot>>;
   createRollbackSnapshot(input: CreateRollbackSnapshotInput): Promise<DesktopResult<AppSnapshot>>;