docs: add runnable Base Bash before-and-after examples

[codeforester]

Summary

• add command-parsing and cleanup before/after examples to the adoption guide
• tie the cleanup example to the executable lifecycle test
• extract and ShellCheck both documentation examples in the docs-example gate

Issue

Fixes #66

Validation

• git diff --check
• ./tests/docs-before-after.sh
• ./tests/docs-contracts.sh
• ./tests/docs-examples.sh

The committed Base Bash v2.2.0 vendor pin is unchanged.