Add automated testing framework (static analysis, unit tests, dry-run template tests)

Introduce a local + CI test suite for cnpg-playground: shellcheck/shfmt static analysis, bats unit tests for the shared bash helpers, and dry-run golden-file tests for the demo YAML rendering. Wire it up as a GitHub Actions check, intended to become required before merging to `main`.

Start by locating the shared Bash helpers, demo YAML rendering entry points, and existing GitHub Actions workflows in the repository. Define the local and CI commands for shellcheck/shfmt, bats, and dry-run golden-file tests, then verify that the check covers all three areas and is suitable for requiring before merges to main.

Written by the indexing model from the issue text.