Shell · ColumnShell's story on the Window & Shell page, the PRD line, stories.json and AGENTS.md (#6)

tasks/ch01-10.md

@@ -102,7 +102,7 @@
 - [x] Page / PageHeader / PageContent
 - [x] AppShell
 - [x] MasterDetail
-- [x] ThreeColumnLayout → `layout::SplitPane`
+- [x] ThreeColumnLayout → `shell::ColumnShell`
 - [x] Dock / DockPanel
 - [x] DockZone / DropIndicator
 - [x] FloatingPanel
@@ -113,6 +113,7 @@
 ## T04 Window & Shell — `src/shell`
 
 - [x] TitleBar
+- [x] ColumnShell / ShellColumn — issue #6: columns to the window's top, each under a header that drags; the traffic lights lead the first, drawn controls lead the first or end the last
 - [x] WindowControls — macOS, Windows, Linux
 - [x] TrafficLights → `shell::WindowControls`
 - [x] WindowDragRegion — `shell::drag_region`: native drag area, Linux move, double-click zoom