Introduces ExecutionEnvEvent enum (15 variants covering lifecycle, Docker
image pull, Daytona snapshot, and git clone operations) with callback
injection on all execution environment implementations. Adds
PipelineEvent::ExecutionEnv wrapper and Setup* variants for setup command
tracking. Wires callbacks in run.rs so exec env and setup events flow
through the pipeline event emitter to stderr and progress.ndjson.
Also changes PipelineEngine::new to accept Arc<EventEmitter> so the
emitter can be shared with exec env callbacks before engine construction.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>