feat: complete working-copies workspace-first cleanup

- Remove clone_mode/branch/new_branch from frontend create session flow.
- Add workspace picker to CreateSessionForm; auto-create default workspace when repo selected.
- Fix tool-starter.tsx and use-start-tool.ts createInstance signatures after API change.
- Remove clone mode badge from SessionCard.
- Delete stale backend unit tests referencing removed clone_mode schema fields.
- Update OpenSpec working-copies tasks and mark change completed.
- Regenerate project maps.

Quality gates: npm run typecheck, npm run lint, npm test -- --run (82 passed),
python3 -m py_compile on changed backend files.

apps/web/src/components/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/web/src/components
 
 ## role
-Provides reusable, foundational React UI components and utilities for the web application, including layout shell, data visualization, navigation guards, and user feedback systems.
+Provides foundational, reusable UI components and utilities for the web application, including layout shell, data states, icon system, code display, routing guards, and toast notification rules.
 ## parent
 index: apps/web/src/.pi-map.index.md
 map: apps/web/src/.pi-map.md