Remove toplevel MANIFEST.in

[rolandwalker]

Description

Remove toplevel MANIFEST.in, which is not used by uv-build for publishing

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Medium: likely sdist regression from deleting MANIFEST.in while still using setuptools backend
   Evidence: build backend is still setuptools ([build-system] build-backend = "setuptools.build_meta") in pyproject.toml:37 and pyproject.toml:38.
   MANIFEST.in previously added non-package files (notably test/**/*.feature, test/**/*.cnf, docs/screenshots). Removing it can drop these from source distributions, which can break downstream “test-from-sdist” packaging flows.
   Action: either restore MANIFEST.in or explicitly replicate required sdist inclusions via setuptools config and verify expected contents.

2. Missing test/guardrail: no artifact-content check after uv build
   Current publish workflow builds artifacts at publish.yml:84 but does not validate sdist contents.
   Action: add a CI step that lists the built tar.gz and asserts required files exist (at minimum license/readme/changelog, plus any intentionally shipped tests/docs).

No direct security issues identified in this PR’s diff.

[rolandwalker]

According to Codex, I don't understand the full implications here.