Forward OPENAI_API_KEY (Forgejo secret) and LLM_ENABLED (Forgejo actions
variable) into the SSH deploy and write them to backend/.env.runtime ONLY
when non-empty, so the LLM chat (#960/#957) stays dormant
(llm_enabled=False, openai_api_key=None) until BOTH are set in Forgejo.
Mirrors the OBJECTIVE_API_KEY wiring (env block + envs list + idempotent
.env.runtime grep-sed-or-printf), but guarded by [ -n ] so an unset
secret/var writes nothing. No literal key (refs only). Compose header
documents both as optional. .env.runtime is shared by backend+worker+beat.
Refs #960