Install an environment listing into your workspace [write]

Creates a copy of the listed environment in the caller's workspace that extends the original, so composition (packages, tool policy, networking) is applied at write time; another workspace's listing is composed from its approved version. A second install of the same listing by the same workspace is idempotent and returns the existing copy. Answers 404 when the shelf is not enabled or the listing is not found. Answers 503 not_migrated until the shelf migration is applied.