feat(workflow): Implement Worker Runtime with Context Shell
- Implemented `ContextShell` trait providing `ls`, `find`, `grep`, `cat`, `patch`, `wc` primitives.
- Added `WorkerContext` struct acting as the shell interface for LLM workers.
- Implemented `get_tool_definitions` to export OpenAI-compatible tool schemas.
- Added `patch` command for atomic, exact-match text replacement.
- Added comprehensive tests covering happy paths and edge cases (ambiguity, unicode, regex errors).
- Updated design documentation.
efd2c42775