chore: remove unused _test_utils.ts and enable --doc check

- Remove src/commands/_test_utils.ts (never imported)
- Update deno.json check task to use --doc for JSDoc validation
- Update documentation to remove reference to deleted file

Author: lambdalisue

.claude/repository_development.md

@@ -11,7 +11,6 @@ Coding conventions and development practices for the Probitas CLI.
 
 **Test Utilities**
 
-- Use `src/commands/_test_utils.ts` for shared test helpers
 - Avoid testing internal details; focus on command input/output behavior
 
 ## Command Implementation Pattern

deno.json

@@ -14,7 +14,7 @@
     ]
   },
   "tasks": {
-    "check": "deno check ./**/*.ts",
+    "check": "deno check --doc",
     "test": "deno test -A --parallel --shuffle",
     "test:coverage": "deno task test --coverage=.coverage",
     "coverage": "deno coverage .coverage --include='src/'",