add markdown linting and cut AGENTS.md back to rules

AGENTS.md was a per-module tour of the code, which restated what the code says
and had already drifted: it described the example as emitting three inputs when
it emits five. It is now the conventions an agent cannot infer -- dora version,
uv, ruff, numpy docstrings, the platform requirement, act, and what not to do --
and nothing else. The gotchas it carried are explained at their point of use in
the code instead.

pymarkdown lints the markdown, configured in pyproject.toml and run in CI
beside ruff. It caught a fenced block with no language and an over-long table
row. ruff's pydocstyle convention moves to numpy.

GEMINI.md is removed.

.gitea/workflows/validate.yml

@@ -69,6 +69,8 @@ jobs:
 
       - run: uv run ruff check .
 
+      - run: uv run pymarkdown scan *.md
+
       - run: uv run pytest -q
 
       # strict_types: true in the dataflow turns a type warning into a non-zero