Reduce lib.rs from 7481 to 6557 lines (-12.4%) by extracting:
- paths.rs: Session/workspace path utilities (get_todo_path, get_logs_dir, etc.)
- streaming_parser.rs: StreamingToolParser for LLM response parsing
- utils.rs: Diff parsing and shell escaping utilities
- webdriver_session.rs: Unified Safari/Chrome WebDriver abstraction
All public APIs preserved via re-exports for backward compatibility.
Added 13 new unit tests across extracted modules.
All 225 tests pass.
