Fixes#27
Changes:
1. Variable uses type instead of interface
2. Add JSON envelope output {type, value} to all CLI var commands
3. Add list method with scope prefix matching to VariableStore and CLI
4. Fix var-db path to default to <storePath>/variables.db instead of <defaultStorePath>/variables.db
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>