Pr 1724 (#1939)

* 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>

packages/coding-agent/docs/keybindings.md

@@ -119,6 +119,13 @@ Modifier combinations: `ctrl+shift+x`, `alt+ctrl+x`, `ctrl+shift+alt+x`, etc.
 | `selectConfirm` | `enter` | Confirm selection |
 | `selectCancel` | `escape`, `ctrl+c` | Cancel selection |
 
+### Tree Navigation
+
+| Action | Default | Description |
+|--------|---------|-------------|
+| `treeFoldOrUp` | `ctrl+left`, `alt+left` | Fold current branch segment, or jump to the previous segment start |
+| `treeUnfoldOrDown` | `ctrl+right`, `alt+right` | Unfold current branch segment, or jump to the next segment start or branch end |
+
 ### Session Picker
 
 | Action | Default | Description |