* feat(coding-agent): add fold/unfold to tree branch navigation
Remap left/right arrow keys in the session tree to jump between branch
segment starts (first child of each branching point). Left on a foldable
node (root or segment start) folds it, hiding its descendants. Right on
a folded node unfolds it. Nested fold state is preserved. Search and
filter changes reset all folds.
Folded branches show ⊞ in the connector (├⊞/└⊞) or as a prefix for
roots. Active path marker • appears after the fold indicator.
Add Page Up/Down support for paging through the tree list, consistent
with session selector and config selector.
* feat(coding-agent): improve tree branch folding closes#1724
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Co-authored-by: Sviatoslav Abakumov <dust.harvesting@gmail.com>
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