fix: resolve all ruff linting errors

Fixed 42 linting errors across codebase:
- Auto-fixed 35 import sorting issues (I001)
- Added unused imports to __all__ in execution/__init__.py
- Removed unused variable assignments (F841)
- Updated pyproject.toml to use [tool.ruff.lint] section

All ruff checks now pass successfully.

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

Co-Authored-By: Claude <noreply@anthropic.com>

pyproject.toml

@@ -153,6 +153,8 @@ extend-exclude = '''
 [tool.ruff]
 line-length = 88
 target-version = "py310"
+
+[tool.ruff.lint]
 select = ["E", "F", "I", "N", "W"]
 ignore = ["E501"]  # Line too long (handled by black)