move pi-mono into companion-cloud as apps/companion-os

- Copy all pi-mono source into apps/companion-os/
- Update Dockerfile to COPY pre-built binary instead of downloading from GitHub Releases
- Update deploy-staging.yml to build pi from source (bun compile) before Docker build
- Add apps/companion-os/** to path triggers
- No more cross-repo dispatch needed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/tui/src/undo-stack.ts

@@ -0,0 +1,28 @@
+/**
+ * Generic undo stack with clone-on-push semantics.
+ *
+ * Stores deep clones of state snapshots. Popped snapshots are returned
+ * directly (no re-cloning) since they are already detached.
+ */
+export class UndoStack<S> {
+  private stack: S[] = [];
+
+  /** Push a deep clone of the given state onto the stack. */
+  push(state: S): void {
+    this.stack.push(structuredClone(state));
+  }
+
+  /** Pop and return the most recent snapshot, or undefined if empty. */
+  pop(): S | undefined {
+    return this.stack.pop();
+  }
+
+  /** Remove all snapshots. */
+  clear(): void {
+    this.stack.length = 0;
+  }
+
+  get length(): number {
+    return this.stack.length;
+  }
+}