[FEATURE]: let humans favourite sessions #47700

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add a way for humans to favourite/star sessions in the session list so important ones stay easy to find.

V1 TUI has pin (session_pin / ctrl+f on the switcher). V2 still has no first-class favourite — sessions stay recency-sorted, so long-running or reference threads are easy to lose.

Related: #42940 (same ask, closed on template, not as wontfix). #36193 is about pinned keyboard shortcuts, not a favourite mark.