Return to the previously-active session when the current one is closed

[olomix]

When I close the active session, agterm selects the next one by position rather than by where I just came from. That leads to unintuitive jumps.

Two examples, starting from sessions 1 2 3 while on session 1:

• Open a new session after the current one → list is 1 4 2 3, focused on 4. Close 4 → I land on 2.
• Open a new session at the end → list is 1 2 3 4, focused on 4. Close 4 → I land on 3.

In both cases I'd expect to return to session 1 — the one I was on before I opened the session I just closed. It's the same "go back to where I came from" model that Ctrl-Tab already uses, and agterm already tracks that navigation history for it.

So the idea: when the active session closes, return to the most-recently-active surviving session instead of the neighbor by position.

I'd be happy to open a PR if you agree on the behavior. My preference is to just make this the behavior (no setting). If you'd rather keep the current positional behavior available, I can add a setting instead — in which case we'd need to decide the default.

[umputun]

agreed, let's make it the default. No setting.

the recency stack ctrl-tab uses is already the right source, and closeSession prunes the closing session from it before it reselects, so the most-recently-active survivor is already at the front. One constraint: scope the MRU pick to the current workspace (and the active focus filter when one is on), since an unscoped "most recent survivor" could pull you into another workspace or silently drop a focus filter on close, which is more disorienting than the positional neighbor.

empty MRU falls straight back to the existing positional reselectionTarget. That covers a fresh restore before anything is activated, sessions that were never focused, or the case where the only recent entry was the one just closed. So nothing regresses, the worst case is exactly today's neighbor behavior, never an empty selection.

if you want to take it, a PR shaped that way would be welcome. RecencyStack.top(1, in:) over the surviving in-scope ids, else reselectionTarget, keeps it a small change in agtermCore with the logic unit-testable.

[olomix]

Opened #231 implementing this as agreed — MRU reselection with no setting, scoped to the closing session's workspace (and the flagged set in flagged sidebar mode), falling back to the positional pick when the recency stack is empty.