feat(command palette): add View toggles (Sidebar, Active Agents, Canvas, Shelf, Show Diff)

[onevcat]

Summary

PR4 of the four-PR command palette plan (doc-onevcat/plans/2026-05-16-command-palette-architecture-plan.md). Surfaces the 5 view toggles that were previously hotkey-only.

Command	Hotkey	Category	Suggested?	Gating
Toggle Sidebar	⌘⌃S	.view	yes	always
Toggle Active Agents Panel	⌘⌥P	.view	yes	always
Toggle Canvas	⌘⌥↩	.view	yes	always
Toggle Shelf	⌘⇧↩	.view	yes	always
Show Diff	⌘⇧Y	.view	yes	hidden unless a worktree is selected

Each is a one-liner through PR3's .appShortcut factory, with keyword aliases for fuzzy search (sidebar, agents, canvas, shelf, diff, etc.).

Architectural change: lift leftSidebarVisibility into AppFeature.State

Previously the main window's sidebar visibility lived as @State in ContentView, mutated via a FocusedValue closure invoked from the menu's "Toggle Left Sidebar" button. The palette can't reach scene-scoped FocusedValue closures from a reducer, so I lifted the state up:

• AppFeature.State.leftSidebarVisibility: NavigationSplitViewVisibility
• New actions: .toggleLeftSidebar, .showLeftSidebar, .setLeftSidebarVisibility(_) (last one backs the NavigationSplitView's columnVisibility binding)
• ContentView constructs a Binding(get:set:) against the store
• Animation moves from withAnimation { ... } (in the imperative closure) to .animation(.easeOut(duration: 0.2), value: store.leftSidebarVisibility) (on the view)
• The diff window keeps its own per-scene columnVisibility — they're independent NavigationSplitViews, so this doesn't affect the diff sidebar's behavior

The FocusedValue closures (toggleLeftSidebarAction, revealInSidebarAction) still exist and now dispatch TCA actions instead of mutating local state — menu behavior unchanged.

Test plan

• make build-app — green
• make check (swift-format + swiftlint strict) — green
• CommandPaletteFeatureTests + AppFeatureCommandPaletteTests + AppShortcutsTests — all pass
• New tests:
  • commandPaletteItems_onlyGlobalWhenEmpty updated to include the 4 new always-on view commands
  • commandPaletteItems_includesShowDiffWhenWorktreeSelected
  • commandPaletteItems_omitsShowDiffWithoutSelectedWorktree
• Manual UI verification — please confirm before merging:
  • Open Cmd+P → see 4 "Toggle X" commands and (when on a worktree) "Show Diff" in Suggested
  • Activate each — sidebar collapses/expands, canvas/shelf/agents panels toggle, diff window opens
  • Trigger ⌘⌃S directly (no palette) — sidebar still toggles via menu/FocusedValue path
  • Open the diff window, hit ⌘⌃S there — diff window's own sidebar toggles independently