Implement the hybrid clipboard with deferred cut

⌘X/⌘C/⌘V for cards and lanes per 04-interactions.md § Clipboard:

- ClipboardStore stages full folder snapshots eagerly at the gesture into
  Application Support (at most the current copy; sweep at launch and on
  each copy purges what the pasteboard no longer references; a copy made
  before quitting pastes whole after restart) and writes the pasteboard a
  JSON manifest — every entry embedding its index.md, lane entries their
  cards' too — plus plain-text titles.
- Cut is Finder-style deferred: items dim in place off pendingCut, void on
  pasteboard takeover (changeCount, no timers), source-board close, or
  per-item external tombstoning; the first armed paste moves the surviving
  originals whole (tombstoned interior cards land in the destination's
  trash), a second paste materializes copies from staging.
- Paste anchors by the shared flatten-order rule (NewCardTarget's anchor,
  extracted); a tombstoned selection never anchors; lane paste reaches the
  right end and stays enabled on a zero-lane board; paste into the source
  board is the within-board lane duplicate; copies keep created, take
  fresh GUIDs, and strip tombstoned cards; trash-sourced copies strip
  deleted: at materialization; ⌘X is disabled on the trash side.
- A degraded paste is loud, never silent: staging gone → the embedded
  index.md fallback lands content-intact, attachments absent, and a
  BannerCenter-phrased row names what was lost.
- The standard Edit items validate through conditionally-attached
  onCommand handlers, so AppKit's enablement mirrors the availability
  predicates; text fields keep their own clipboard while focused.

879 unit tests (68 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/App/AppModel.swift

@@ -176,6 +176,19 @@ public final class AppModel {
     /// concern, and nothing outside this module has any business reaching into a gesture in flight.
     let dragSession = DragSession()
 
+    /// The app's one clipboard (04-interactions.md ▸ Clipboard).
+    ///
+    /// App-wide for the drag session's reason turned up a level: a cut/copy **outlives the board it
+    /// came from** — the pasteboard and the staged snapshot survive the source window closing, and
+    /// survive the app quitting — so nothing board-scoped could own it. It lives here rather than as
+    /// a singleton for `styleRecents`' reason (a test holds its own rather than colliding with the
+    /// app's, which for this one also means staying off the machine's real pasteboard), and every
+    /// board window reaches it through the environment.
+    ///
+    /// Building it here is also the **launch sweep** (04: "a sweep at launch"): the store's `init`
+    /// reads the pasteboard once and collects every staged tree it no longer names.
+    public let clipboard = ClipboardStore()
+
     // MARK: Sessions
 
     /// One open board window and everything hanging off it.