Improve right-sidebar edge peek discoverability and add a setting

Teach the edge-hover gesture via the existing toggle tooltip and a
one-shot first-close coachmark, and make Arc-style edge peek opt-out
in Appearance settings (default on).

src/shared/constants.ts

@@ -275,6 +275,8 @@ export function getDefaultSettings(homedir: string): GlobalSettings {
     experimentalNativeChat: false,
     openInApplications: [...DEFAULT_OPEN_IN_APPLICATIONS],
     rightSidebarOpenByDefault: true,
+    rightSidebarEdgePeekEnabled: true,
+    rightSidebarEdgePeekTipDismissed: false,
     showGitIgnoredFiles: true,
     sourceControlViewMode: 'list',
     sourceControlGroupOrder: DEFAULT_SOURCE_CONTROL_GROUP_ORDER,

src/shared/types.ts

@@ -2642,6 +2642,14 @@ export type GlobalSettings = {
   openInApplications?: OpenInApplication[]
   /** Deprecated: migration/backward-compat only. Use PersistedUIState.rightSidebarOpen. */
   rightSidebarOpenByDefault: boolean
+  /** When enabled (default), hovering near the window's right edge while the
+   *  right sidebar is closed peeks it as a floating overlay. Opt-out for users
+   *  who only want the explicit toggle/shortcut. Optional for legacy profiles;
+   *  readers treat absent as true. */
+  rightSidebarEdgePeekEnabled?: boolean
+  /** One-shot coachmark after first closing the right sidebar while edge peek
+   *  is enabled. Absent/false means the tip can still show. */
+  rightSidebarEdgePeekTipDismissed?: boolean
   showGitIgnoredFiles?: boolean
   /** Preferred Source Control changes layout. Per-user, not per-workspace. */
   sourceControlViewMode: SourceControlViewMode