improve: fix Avro nullable fields, remove dead npm-publish workflow, add tests

[Coding-Dev-Tools]

Automated improvement by dev-engineer

Changes:

• Fixed AvroWriter to properly handle nullable fields (detects nulls across all rows, not just first row)
• Removed dead workflow (datamorph is Python-only, no package.json)
• Updated publish.yml with modern actions (actions/checkout@v4, setup-python@v5, pypa/gh-action-pypi-publish@release/v1)
• Added ruff linting step to publish workflow
• Fixed package.json version to match Python package (0.1.1)
• Removed unused src/datamorph/main.py
• Added comprehensive test coverage for Avro nullable fields, empty datasets, batch CLI options, and type inference edge cases
• Fixed ruff F402 lint error (shadowed loop variable in AvroWriter)
• Added AGENTS.md for agent discoverability
• Updated CHANGELOG.md with unreleased changes