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

---------

Co-authored-by: Sviatoslav Abakumov <dust.harvesting@gmail.com>

packages/tui/src/keybindings.ts

@@ -44,6 +44,9 @@ export type EditorAction =
 	| "undo"
 	// Tool output
 	| "expandTools"
+	// Tree navigation
+	| "treeFoldOrUp"
+	| "treeUnfoldOrDown"
 	// Session
 	| "toggleSessionPath"
 	| "toggleSessionSort"
@@ -105,6 +108,9 @@ export const DEFAULT_EDITOR_KEYBINDINGS: Required<EditorKeybindingsConfig> = {
 	undo: "ctrl+-",
 	// Tool output
 	expandTools: "ctrl+o",
+	// Tree navigation
+	treeFoldOrUp: ["ctrl+left", "alt+left"],
+	treeUnfoldOrDown: ["ctrl+right", "alt+right"],
 	// Session
 	toggleSessionPath: "ctrl+p",
 	toggleSessionSort: "ctrl+s",