fix: resolve CI linting failures

- Removed unused imports (pathlib.Path, Schema, EnvVar, Optional)
- Fixed f-strings without placeholders in generator.py and validators.py
- Removed unused variables in generator.py
- Updated CI workflow to use correct project name (envschema)

envschema/core.py

@@ -3,7 +3,7 @@
 from dataclasses import dataclass, field
 from typing import Optional
 
-from envschema.schema import Schema, EnvVar
+from envschema.schema import Schema
 from envschema.loader import EnvLoader
 from envschema.validators import validate_value