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.

dataflow.yml

@@ -12,6 +12,11 @@
 # wires the viewer from a tag instead -- see README.md.
 #
 # Run it with `--uv` so every node gets its own managed environment.
+# strict_types makes a type warning a non-zero exit. It also means a *typed*
+# manifest input must be wired below: `dora validate --strict` warns
+# `input_types key "..." not found in inputs list` for any it leaves unwired, so
+# a new typed input on the node has to be driven from tests/source.py with a
+# matching entry in `output_types` here, or left untyped.
 strict_types: true
 
 nodes: