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.

AGENTS.md

@@ -131,6 +131,7 @@ src/
 2. NPCs without a `[combat]` section get default stats (20hp/4atk/2def/5xp)
 3. Room IDs are `<region_dir>:<filename_stem>`
 4. Cross-region exits work — just reference the full ID
+5. Run `mudtool validate -w ./world` to check schemas, references, and values before committing
 
 ### New DB table
 1. Add `CREATE TABLE IF NOT EXISTS` in `SqliteDb::open()`