UI: add auto-opening and -selection in exec sidebar

- [ ] if there’s only one task group, it should auto-open
- [ ] if there’s only one task group and one task in it, it should be auto-selected

Start by locating the exec sidebar implementation and the entry points that build task groups and select tasks. Verify the behavior with one task group, and with one group containing one task; done means the sole group opens automatically and the sole task is selected automatically.

Written by the indexing model from the issue text.