Update documentation and CI to include world validation checks

- Added a note in AGENTS.md about using `mudtool validate -w ./world` for schema validation before committing.
- Updated TESTING.md to include a checklist item for the new validation command.
- Modified smoke-tests.yml to run the world validation command as part of the CI workflow.

TESTING.md

@@ -218,6 +218,7 @@ Run through the checks below before every commit to ensure consistent feature co
 - [ ] Existing players can still log in when registration is closed
 
 ## MUD Tool - CLI
+- [ ] `mudtool validate -w ./world` checks world data (schemas, references, values)
 - [ ] `mudtool players list` shows all players
 - [ ] `mudtool players show <name>` shows details
 - [ ] `mudtool players set-admin <name> true` works