The two env-reuse helpers (`applyExistingEnvironment` and
`applyExistingEnvironmentRuntimeMetadata`) were always called together
and shared the same intent: carry the previous TaskEnvironment forward
into the launch request. Merge them into a single
`reuseExistingEnvironment(ctx, req, env)` so the executor-type gate
runs once for both env-level fields and runtime metadata, and callers
have a single entry point.
Document the layered metadata rule (running wins for container_id;
env wins for sprite_name and other stable IDs) inline so the next
reader doesn't have to reverse-engineer it from two functions.
José Almeida
470c545bf9