What changed:
- Added file_writer stage support in nightshift/config.py:93, nightshift/agents.py:446, and nightshift/pipeline.py:374.
- Added file-block parsing and deterministic patch generation in nightshift/patches.py:65.
- file_writer agents now return complete file blocks:
```file:relative/path.py
<complete file content>
```
NightShift reads current files and generates the unified diff itself.
- Existing patch_validator, patch_normalizer, and patch_apply still run after that.
- Switched tiny-lisp-nightshift/nightshift.yaml:51 to type: file_writer.
- Updated tiny-lisp-nightshift/agents/implementer.md:1 to request file blocks instead of diffs.
- Updated README, tutorial, config reference, and design doc.
Also added tests for parsing file blocks, generating multi-file diffs, validating those diffs, and running a full file_writer -> normalize -> validate pipeline.
Verification: python -m unittest discover -v passes, 101 tests.
