[New Integration] Buildkite

## Description

[Buildkite](https://buildkite.com/) is a CI/CD platform. Organization-level audit log events capture administrative and security-sensitive actions: pipeline lifecycle, user and team management, secret access and configuration, SSO, API token management, cluster/agent token changes, user account security events, Test Engine suite lifecycle, package registry lifecycle, and related governance activity.

The use cases supported include monitoring pipeline, secret and token changes; user and team membership/access changes; and correlation with other telemetry in Elastic Security.
Audit events can be fetched via the Buildkite GraphQL API.

## Architecture
Elastic Agent cel input polls the Buildkite GraphQL endpoint `https://graphql.buildkite.com/v1`.
The integration collects all organization audit event types surfaced by the API (**~150 `AuditEventType` values** in one **`buildkite.audit`** stream), including at minimum these **categories** (non-exhaustive):

- **Organization management** — members, invitations, settings
- **Pipeline management** — create/update/delete, schedules, visibility, webhooks
- **Secrets** — create/update/delete; **reads** (**`SECRET_READ`**)
- **Teams** — membership and resource access
- **SSO** — provider lifecycle and configuration
- **API tokens** — create/revoke/update and org access changes
- **Clusters and agents** — cluster/queue/token and permission changes
- **User account** — password reset/TOTP/email changes
- **Test Engine** — suite lifecycle events
- **Package registries** — registry lifecycle events

## References

- [Buildkite GraphQL API](https://buildkite.com/docs/apis/graphql-api)
- [Audit log](https://buildkite.com/docs/platform/audit-log)