Use buildkite-agent meta-data (set, set-batch, get, exists, keys) to pass small string key-value data between steps and jobs within the same build.

Documented steps

1. Set a value: `buildkite-agent meta-data set <key> [value]` — value can be a CLI argument, piped stdin, or file redirect, e.g. `buildkite-agent meta-data set "release-version" "1.1"`. Value must be a non-empty, non-whitespace-only string. Doc: https://buildkite.com/docs/agent/cli/reference/meta-data
2. Set multiple pairs in one request: `buildkite-agent meta-data set-batch <key=value>...`, e.g. `buildkite-agent meta-data set-batch foo=bar "greeting=hello world"`.
3. Get a value: `buildkite-agent meta-data get <key> [--default <value>] [--build <build-id>]`. Errors if the key isn't set and no `--default` is given; `--build` reads another build's meta-data and takes precedence over `--job`.
4. Check existence: `buildkite-agent meta-data exists <key>` exits 0 if set, 100 if not set. List all set keys with `buildkite-agent meta-data keys`.
5. Meta-data is stored at the build level (shared across all jobs in that build), not per job; if multiple jobs set the same key, the build keeps whichever value was written last. Doc: https://buildkite.com/docs/pipelines/configure/build-meta-data
6. Ordering constraint: a value can only be read after it has been set — use a `wait` step, or otherwise guarantee step ordering, so steps that read meta-data run after the step that sets it.
7. Once set, a meta-data value cannot be deleted, only overwritten via `set`. Each value is capped at 100KB, though values over 1KB are discouraged — use build artifacts for larger data.
8. The special read-only key `buildkite:webhook` returns the JSON body of the webhook that triggered the build (only for webhook-triggered builds, cached roughly 7 days), e.g. `buildkite-agent meta-data get buildkite:webhook`. Doc: https://buildkite.com/docs/pipelines/configure/build-meta-data