agent v0.36.3: DetachDrive loop-umounts stacked binds (full detach)

Detach now removes ALL stacked binds at a stable path, not just one layer, so an
eject fully detaches even with >1 bind (keeping fail-close intact).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/intermediary.go

@@ -293,13 +293,18 @@ func (b *GuestBinder) DetachDrive(ctx context.Context, where string) error {
 	if stable == "" {
 		return fmt.Errorf("guest-detach: %q is not a /mnt/<name> mount", where)
 	}
-	if !isHostMountpoint(stable) {
-		return nil // already detached
+	// Loop-umount: a stable path can carry MORE THAN ONE stacked bind (e.g. an operator-applied bind on
+	// top of the agent's, or a rare attach race). Detach must remove ALL layers, else eject leaves a
+	// lower bind exposing data → fail-close broken. Bounded to avoid an infinite loop.
+	for i := 0; i < 16 && isHostMountpoint(stable); i++ {
+		if err := b.run(ctx, "umount", stable); err != nil {
+			return fmt.Errorf("guest-detach: umount %s (layer %d): %w", stable, i, err)
+		}
 	}
-	if err := b.run(ctx, "umount", stable); err != nil {
-		return fmt.Errorf("guest-detach: umount %s: %w", stable, err)
+	if isHostMountpoint(stable) {
+		return fmt.Errorf("guest-detach: %s still a mountpoint after 16 umounts", stable)
 	}
-	b.logger.Info("guest-detach: drive unmounted from shared parent (live, fail-closed)", "where", where, "stable", stable)
+	b.logger.Info("guest-detach: drive fully unmounted from shared parent (live, fail-closed)", "where", where, "stable", stable)
 	return nil
 }