build with uv rather than pip

The build command was `pip install .`, which is what the dora docs use because
`--uv` rewrites it. Spelling it `uv pip install .` means the dataflow never
installs through pip at all, whether or not `--uv` was passed. The cost is that
a node then needs a virtualenv to install into, so dataflows must be run with
`--uv`; AGENTS.md says so.

Note that the PR #3 branch still carries `pip install .`; landing this branch
on top of it is what corrects that.

AGENTS.md

@@ -6,7 +6,8 @@ Rules for AI coding assistants working in this repository.
   `.gitea/workflows/validate.yml`.
 - **`uv` for everything:** `uv sync --group dev`, `uv run pytest`,
   `uv run ruff check .`, `uv run python ...`. Never `pip` into the ambient
-  environment.
+  environment. A node's `build:` is `uv pip install .`, so dataflows must be run
+  with `--uv`.
 - **`ruff` is the linter and the formatter.** `uv run ruff check .` must be clean.
 - **Markdown is linted too.** `uv run pymarkdown scan *.md` must be clean. Its line
   length is set in `pyproject.toml`.