Extended ThingTemplate with optional container fields (capacity, closed, locked).
When a template includes capacity, spawn_thing now creates a Container instead
of a regular Thing.
Added two example container templates:
- chest.toml: non-portable, capacity 5, starts closed
- sack.toml: portable, capacity 3, starts open
ThingTemplate dataclass mirrors MobTemplate pattern. load_thing_template
and load_thing_templates parse TOML files from content/things/. spawn_thing
creates Thing instances from templates. Includes rock and fountain examples.