Check root Python script formatting in CI (#25165)

## Why

Python files under `scripts/` were not covered by the repository
formatting recipe or the CI formatting job, so formatting drift could
merge unnoticed.

## What

- Add a dedicated `scripts/pyproject.toml` and `scripts/uv.lock` so
root-script formatting uses a locked Ruff version.
- Extend `just fmt` to format root Python scripts and add
`fmt-scripts-check` for CI.
- Run `just fmt-scripts-check` from `.github/workflows/ci.yml`,
installing `uv` through SHA-pinned `astral-sh/setup-uv` while retaining
the `uv` `0.11.3` pin.
- Apply Ruff formatting to the root Python scripts, including
`scripts/just-shell.py`, and extend
`sdk/python/tests/test_artifact_workflow_and_binaries.py` to cover the
root formatting recipe.
- Update `AGENTS.md` so agents run `just fmt` after code changes
anywhere in the repository.

## Validation

- Extended the existing Python SDK workflow test to assert that `just
fmt` includes root Python scripts.

scripts/pyproject.toml

@@ -0,0 +1,9 @@
+[project]
+name = "codex-scripts"
+version = "0.0.0"
+requires-python = ">=3.10"
+dependencies = ["ruff>=0.15.8"]
+
+[tool.uv]
+exclude-newer = "7 days"
+index-strategy = "first-index"