Promote to environment

Promotes the source environment’s current content + code into this environment. Runs the unified deployment in-process: it writes the env-keyed template_snapshot, atomically commits the env pointer, AND dispatches the managed code build — this CLOSES the env-pin → code-deploy gap (a promote now deploys code, not just content). Returns 202 with the created deployment row; the snapshot + env-pointer commit are already done, while the build runs asynchronously — poll GET /v1/deployments/{deployment_id} to watch code_status reach deployed. The default environment cannot be a promote target.