refactor(tito): remove doctor alias, use health as single command

Remove the doctor/health alias duplication in favor of a single
'tito system health' command for environment validation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

tinytorch/tests/cli/test_cli_execution.py

@@ -95,7 +95,7 @@ class TestCommandExecution:
 
     @pytest.mark.parametrize("command,subcommand", [
         ('system', 'info'),
-        ('system', 'doctor'),
+        ('system', 'health'),
         ('module', 'status'),
         ('module', 'list'),
         ('community', 'join'),