Plug emailgen into the systemd-supervised fleet:
- New deploy/decnet-emailgen.service.j2 mirroring decnet-orchestrator's
shape: simple service, restart-on-failure, docker supplementary group
(driver shells `docker exec` to drop EMLs into the spool), the same
hardening directives as the rest of the fleet.
- decnet.target now Wants both decnet-emailgen.service and
decnet-orchestrator.service. Orchestrator's absence from the target
was a historical oversight — fixing it here while the file is open.
`decnet init` already globs deploy/decnet-*.service.j2 so the new unit
ships automatically; no init-side change needed. Emailgen-specific env
knobs (DECNET_EMAILGEN_LLM, _MODEL, _PERSONAS, _TIMEOUT) are documented
in the unit and operator-tunable via /opt/decnet/.env.local.
