CI: smoke test for kenney-assets install CLI

[dannystaple]

Context

Part of #69 (CI validating real activity code). Unlike most of coder_dojo_common, the Kenney asset installer is a self-contained utility that doesn't need an external activity sheet to test meaningfully — it can be smoke-tested on its own.

The CLI entry point is kenney-assets (coder_dojo_common.cli:kenney), with two subcommands:

• kenney-assets query — lists assets and download/install state (coder_dojo_common/cli.py)
• kenney-assets install <ASSET_NAME> — downloads (if not cached) and extracts the asset zip into ./assets/<ASSET_NAME>/ (coder_dojo_common/cli.py, backed by KenneyAssets/get_assets/get_download_link in coder_dojo_common/kenney_assets.py)

Task

Add a CI job (GitHub Actions) that:

• Installs the package (poetry install, as the other workflows already do)
• Picks one small, stable Kenney asset (small file size, unlikely to be renamed/removed from kenney.nl) and runs kenney-assets install <ASSET_NAME> against it
• Asserts the install succeeded: assets/<ASSET_NAME>/ exists and is non-empty (e.g. contains at least one extracted file)
• Optionally also runs kenney-assets query and checks it exits cleanly and lists the asset

Notes / open questions

• This hits the live kenney.nl site, so the job will be a bit network-flaky by nature — decide whether to continue-on-error or just accept occasional retries, and pick the smallest/most stable asset available to minimise both download time and risk of it disappearing
• Could run this as its own workflow or as an extra job in on-pr-lint.yml — probably worth keeping separate so a flaky network hit doesn't block unrelated lint failures
• If this works well it's a good template for the other Add CI that validates against real activity code #69 sub-areas (games/stats/maths viz), even though those need real activity code from Identify and reference the source activity sheets/repos #68 first