fix(ai-term): keep the F9 toggle reversible in a 3+ window layout

In a layout where the agent had its own split window (e.g. code on top, a
working window, and the agent below), toggling the agent off deleted its
window correctly, but toggling back on reused the working window at the edge
-- displacing its buffer and collapsing three windows to two. The slot-reuse
that avoids a third window on a fresh show was firing on a re-show after the
agent's own window was already deleted.

Flag the toggle-off that deletes the agent's own window; on the next
toggle-on, reuse-edge-window consumes the flag and falls through to a fresh
re-split, so the agent returns to its own window and the other windows are
untouched. The flag only changes the 3+ window case -- after a delete in a
2-window slot-reuse layout one window remains, where re-split and reuse-edge
already coincide, so the existing reuse-edge tests are unaffected.

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