fix: surface actual session branch in frontend

[areycruzer]

Summary

• expose the persisted session branch as a display-safe branch field on SessionView
• keep operational session metadata private while preserving the existing frontend synthetic fallback
• regenerate the OpenAPI spec and frontend schema, with backend/frontend coverage for the branch mapping

Fixes #356

Notes

• Rebase risk: feat(status): collapse session status to five glanceable states (#332) #349 is open and also touches frontend/src/renderer/hooks/useWorkspaceQuery.ts and frontend/src/renderer/hooks/useWorkspaceQuery.test.tsx, so whichever PR merges second may need a small rebase.
• Test scope: this adds controller coverage for the list-session wire shape plus frontend mapper coverage. It does not add a full spawn-persist-read integration test; the shared sessionView helper also serves single-session responses, so the untested surface is small.

Tests

• cd backend && go test ./...
• npm --prefix frontend run test -- src/renderer/hooks/useWorkspaceQuery.test.tsx
• npm --prefix frontend run typecheck