Restore pasting and rename sections to workspaces

Replacing the main menu dropped the Edit menu that AppKit routes
Cmd+V/C/A/Z through, so the composer could not paste at all. The menu is
rebuilt with those items, which --diag now dumps so the wiring can be
checked without a keyboard.

Workspaces also gained an order: the Cmd+K switcher has up/down controls,
creating one opens a naming modal instead of prefilling the composer, and
the UI now says workspace throughout. The stored JSON keeps the original
`sections` keys so existing data and sync payloads stay valid.

AGENTS.md

@@ -156,6 +156,14 @@ brew upgrade --cask prompt
 
 ## Gotchas
 
+- **Setting `NSApp.mainMenu` replaces SwiftUI's default menu.** SwiftUI installs
+  a menu containing the standard Edit items, and AppKit routes Cmd+C/V/X/A/Z
+  through those items — so a hand-built menu with only Hide/Quit silently
+  breaks pasting, copying and select-all in the composer. `setupMainMenu()`
+  therefore rebuilds an Edit menu with `undo:`/`redo:`/`cut:`/`copy:`/`paste:`/
+  `selectAll:` (nil targets, so they travel the responder chain to the field
+  editor). `Prompt --diag` dumps the menu so this can be checked without a
+  keyboard.
 - **Never bind bare keys as `.keyboardShortcut` in the panel** (Space, Delete,
   plain Cmd+C). SwiftUI key equivalents are matched before text input and
   would steal characters from the composer TextField. Only Cmd-modified