Safety fixes:
- blender_clear_scene refuses to wipe a non-empty scene without force=True
- blender_export_stl / import_stl / save_blend resolve relative paths
  against the MCP server's cwd (was leaking Blender's read-only install dir)
- printable-install-addon cleans up stray install.py/addon.py left by
  previous "Install from Disk" mistakes