Converts the bats-support / bats-assert test helpers from git submodules to plain vendored files under tests/helpers/.

Why

The submodules used SSH URLs (git@github.com:bats-core/...), so a fresh or HTTPS clone got empty tests/helpers/ dirs and every bats test failed at load test_helper until someone ran git submodule update --init. Silent, confusing friction. These libs are tiny, MIT/CC0-licensed, and change ~once a year — the textbook case for vendoring. Clones now just work.

What changed

• Vendored each helper trimmed to its essentials — load.bash + src/ + LICENSE (dropped the upstream .github/, test/, CHANGELOG, package.json, etc. — load.bash only sources its own src/). 19 files total.
• Deleted .gitmodules; removed the submodule gitlinks.
• .pre-commit-config.yaml: added top-level exclude: ^tests/helpers/ so our hooks never rewrite vendored upstream code (trailing-whitespace tried to).
• ci-cd.yml: dropped the now-pointless submodules: true from the Lint job checkout.

Verification

• Full bats suite: 129 passing with the trimmed vendored helpers
• No submodule remnants (.gitmodules, .git/modules/tests, inner .git all gone)

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