[CI] Integrate BATS shell tests into main CI workflow

[couimet]

Parent: #657
Blocked by: couimet/github-actions#1

Once couimet/github-actions publishes a BATS composite action, integrate the shell tests currently in .github/workflows/test-shell-scripts.yml as a shell-tests job in .github/workflows/ci.yml and remove the standalone workflow.

[couimet]

The shared BATS action is now implemented in couimet/github-actions#8 — the bats-test composite action. Use it as the reference for this migration: once #8 merges, replace the inline bats-core/bats-action step in .github/workflows/test-shell-scripts.yml with couimet/github-actions/bats-test@main (test-directory: tests/shell) and fold it into the shell-tests job in ci.yml as planned. The shared action wraps the same SHA-pinned bats-core/bats-action@4.0.0 with helper installs disabled, so the migration is a drop-in.