migrate mosaic home to xdg config path

bin/mosaic-prune-legacy-runtime

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-MOSAIC_HOME="${MOSAIC_HOME:-$HOME/.mosaic}"
+MOSAIC_HOME="${MOSAIC_HOME:-$HOME/.config/mosaic}"
 RUNTIME="claude"
 APPLY=0
 
@@ -10,7 +10,7 @@ usage() {
 Usage: $(basename "$0") [options]
 
 Remove legacy runtime files that were preserved as *.mosaic-bak-* after Mosaic linking.
-Only removes backups when the active file is a symlink to ~/.mosaic.
+Only removes backups when the active file is a symlink to ~/.config/mosaic.
 
 Options:
   --runtime <name>   Runtime to prune (default: claude)