Extracts the docker-exec-with-base64-stdin pattern out of canary/planter
and orchestrator/drivers/ssh into a shared decnet.decky_io package.
Both consumers now delegate; the canary planter test still proves the
contract end-to-end.
Adds POST/DELETE /api/v1/deckies/files for arbitrary file drops.
Container resolution is shared with the canary path: topology_id absent
means fleet (<name>-ssh), present routes through resolve_decky_container
which picks <name>-ssh when the topology decky exposes ssh, else the
topology base container decnet_t_<id8>_<name>.
Path validation rejects relative paths and '..' traversal at the request
model layer. Bad base64 → 400; unknown topology → 404; decky not in
topology → 422; docker exec failure → 409.
Topology deploys now plant the configured canary baseline set on every
decky in the topology, mirroring the fleet-deploy hook. Containers are
resolved via resolve_topology_container — <decky>-ssh when the decky
exposes an ssh service, else the topology base container
decnet_t_<id8>_<decky>.
The planter's plant/revoke/seed_baseline grow an optional container=
kwarg; default preserves the fleet <name>-ssh resolution.
