Declare a real artifacts entry to make basectl setup observable

[codeforester]

Context

base_manifest.yaml declares artifacts: [] — the README explains this avoids unnecessary installs. As a result, basectl setup runs but produces no visible artifact reconciliation output, and the manifest contract map lists artifacts → basectl setup with nothing to show for it.

This is the only manifest field in the contract map that produces no observable effect during the demo. A learner cannot understand what artifacts does or why they would use it.

Scope

• Declare at least one artifact in base_manifest.yaml. The best candidate is the existing base_demo_cli Python package in lib/python/: declaring it as a python-package artifact means basectl setup installs it into the project virtual environment, which is directly observable.
• Alternatively, declare a Homebrew tool from the artifact registry (e.g. jq) if that better fits the demo story.
• Update the README Manifest Contract Map table to show the declared artifact and the basectl setup output it produces.
• Update tests/validate.sh to assert at least one artifact is declared (the list is non-empty).
• If the artifact changes CI setup time materially, note that in the PR.

Validation

• basectl setup base-demo --dry-run output lists the declared artifact.
• basectl setup base-demo installs the artifact into the project venv.
• grep -A2 'artifacts:' base_manifest.yaml shows a non-empty list.
• ./tests/validate.sh

[codeforester]

Closing as already resolved by PR #105. base_manifest.yaml now declares a non-empty artifacts list with the bats-core tool artifact, and README/demo/validation coverage make the setup artifact behavior visible.