Change Command History shortcut from ⌘H to ⇧⌘H

⌘H is reserved by macOS for "Hide Application" and pre-empts app-level
menu bindings before they ever fire, so the Command History shortcut
never actually worked. Moved to ⇧⌘H and updated all references (in-app
help, macOS Help Book, README).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/oAIApp.swift

@@ -132,7 +132,7 @@ struct oAIApp: App {
                 Divider()
 
                 Button("Command History") { chatViewModel.showHistory = true }
-                    .keyboardShortcut("h", modifiers: .command)
+                    .keyboardShortcut("h", modifiers: [.command, .shift])
 
                 Button("In-App Help") { chatViewModel.showHelp = true }
                     .keyboardShortcut("/", modifiers: .command)