chore(ci): Check feature manifests and shell scripts

[leocavalcante]

Summary

• .github/workflows/ci.yml runs on pull requests and on pushes to main.
• It checks each devcontainer-feature.json with jq (id, version, and name), runs shellcheck on features/*/install.sh, and runs bash tests/*.sh when that directory has scripts.
• main has no shell test yet, so that step exits 0 and prints that tests/ is empty. Publishing stays in .github/workflows/features.yml. This workflow does not build devcontainer images.

Test plan

• PyYAML loaded .github/workflows/ci.yml
• jq -e '.id and .version and .name' on features/install-php-extensions/devcontainer-feature.json
• shellcheck -x features/install-php-extensions/install.sh (Nix nixpkgs#shellcheck)
• The shell-test step prints No shell tests in tests/. and exits 0
• The Actions run on this pull request is green, with the same three steps