tui: persist agent/model per session

[ry2009]

• Per-session agent/model prefs are persisted and re-applied on every switch (old and new sessions).
• Prefs are saved when leaving a session and applied (or seeded) when entering the next session.
• New pref store file session-pref.json under state dir; unit test for round-trip.

[ry2009]

I know this still may be in talks about approach/want of this feature but thought I would get ahead as I saw it as low hanging fruit

[rekram1-node]

/review

[rekram1-node]

Yea