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.

openspec/changes/.pi-map.md

@@ -4,10 +4,10 @@ dir: openspec/changes
 index: openspec/changes/.pi-map.index.md
 
 ## role
-Tracks and manages specification changes/versions for OpenAPI documents
+Manages change tracking, versioning, and audit logging for OpenSpec schema or configuration modifications.
 ## files
 ## arch
-Simple data structure package with record types for change metadata, likely used by diff/merge tooling
+Event-sourced or changelog-based architecture with immutable change records, likely supporting rollback, diff computation, and history querying patterns.
 ## tags
 -
 ## symbols