Wait for the interrupted operation to clean up before exit (closes #159)

On SIGINT/SIGTERM the process could exit before the interrupted command cleanup defers ran, leaving decrypted data in the temp directory (the blob cache and the decrypted snapshot database).

RunApp now mirrors fx run sequence: start, block on app.Wait(), then app.Stop(), returning only after Stop completes. fx delivers both an OS interrupt and the finished operation Shutdowner.Shutdown() on one channel. Stop runs the OnStop hooks; the operation hook cancels the command and waits for its goroutine to return (bounded by shutdownTimeout) before exit. The old code returned as soon as app.Done fired, without Stop, so a real interrupt unwound to os.Exit while cleanup still ran. Restore loops check the context between chunks and blobs so the wait ends promptly. A cli test drives RunApp through the OnStop hook.

Model: opus-4-8

internal/vaultik/vaultik.go

@@ -128,6 +128,40 @@ func (v *Vaultik) Cancel() {
 	v.cancel()
 }
 
+// StartOperation runs fn in its own goroutine and returns a stop
+// function. fn is the command being run (a restore, verify, prune, and
+// so on); it observes cancellation through the Vaultik context and
+// removes its decrypted scratch files (the blob cache and the temporary
+// snapshot database) from the temp directory as it unwinds.
+//
+// Calling stop cancels the Vaultik context and then blocks until fn has
+// returned — so that unwinding, and the cleanup it does, completes
+// before the caller proceeds — or until the passed context is done,
+// whichever comes first. It reports whether fn returned before that
+// deadline. A signal-driven shutdown must call stop before the process
+// exits; otherwise the process can exit mid-operation and leave
+// decrypted data behind.
+func (v *Vaultik) StartOperation(fn func()) func(context.Context) bool {
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+
+		fn()
+	}()
+
+	return func(ctx context.Context) bool {
+		v.Cancel()
+
+		select {
+		case <-done:
+			return true
+		case <-ctx.Done():
+			return false
+		}
+	}
+}
+
 // CanDecrypt returns true if this Vaultik instance has decryption capabilities
 func (v *Vaultik) CanDecrypt() bool {
 	return v.Config.AgeSecretKey != ""