Add Buildkite agent lifecycle hooks to run custom logic at checkout and command stages of a job

Steps

1. Decide the hook scope: agent-level hooks (apply to every job on that agent, stored in the agent's hooks path) versus per-repository hooks checked into a .buildkite/hooks directory.
2. Create checkout-related hook scripts (pre-checkout, checkout, post-checkout) for logic that must run before or around the repository clone; these execute only in the checkout phase/container.
3. Create command-related hook scripts (pre-command, command, post-command) for logic around the actual build/test/deploy command; these execute only in the command phase/container(s).
4. Use the environment hook — executed both in the checkout container and in each command container — to export variables needed across phases, since variables set in checkout hooks don't automatically carry into command hooks.
5. Test hook exit-code behavior: a failing pre-command (or earlier) hook aborts the job before the command runs, while a failing post-command or pre-exit hook overrides the job's exit code even if the command itself succeeded.