CD CDEvent need a deployment.started and deployment.finished #248

While service.deployed is conceptually similar to artifact.packaged (representing a completed action), there is currently no way to represent the lifecycle of a deployment in progress. This is a gap in the event model that impacts observability, symmetry, and real-time status visibility in CD workflows.

In the CI domain, build.queued, build.started, and build.finished events represent the full lifecycle of a build. For artifact.packaged and artifact.published capture output/resulting state.

However in the CD side, service.deployed represents a completed state like packaged, and service.published aligns with artifact.published, but there's no deployment.started / deployment.finished equivalent to model the deploy process itself.