ci: test released base-cli compatibility - #13

Summary

• Add minimum/latest released Base-CLI compatibility jobs for Python 3.10 and 3.13.
• Build and install the demo wheel before running the compatibility suite.
• Execute every README northstar command and validate record/envelope JSON.
• Add a manual, explicitly non-blocking upcoming-build check.

Validation

• ./tests/validate.sh
• UV_CACHE_DIR=/private/tmp/base-cli-demo-issue5-uv uv run --extra dev pytest -q (14 passed)
• Installed wheel smoke with base-cli==0.4.3: 14 passed
• UV_CACHE_DIR=/private/tmp/base-cli-demo-issue5-uv uv run --with ruff ruff check src tests
• UV_CACHE_DIR=/private/tmp/base-cli-demo-issue5-uv uv run --with ruff ruff format --check src tests
• Compatibility workflow YAML parsed with Ruby YAML loader

Fixes #5