[codex] Add precipitation anomaly diagnostics lab

[stacknil]

Summary

• Add a public-safe precipitation anomaly diagnostics mini-lab under projects/precipitation-anomaly-diagnostics.
• Add synthetic-data tests for preprocessing and EOF behavior, plus a focused GitHub Actions workflow and Dependabot entry.
• Link the mini-lab from the repository README and keep the data/reproducibility/privacy boundaries explicit.

Validation

• python -m pytest in projects/precipitation-anomaly-diagnostics (6 passed, local Python 3.14)
• python -m compileall src scripts
• python -m ruff check .
• CLI help smoke tests for all three project scripts
• git diff --cached --check
• Privacy scan for real institution/person/local-path/secrets patterns; only generic exclusion-policy text matched
• PNG metadata scan for text/EXIF chunks; none found

Notes

• The source material was copied only from the sanitized public export tree. Raw course files, raw datasets, and private working artifacts were not added.