fix(config): delete the model-routing chain — nothing has called it since April (#119)

Closes #116.

Deletes config/models.yaml, internal/config/models.go (LoadModels, ModelFor),
its test, the ModelsFile field on Config, and SUPERVISOR_MODELS_FILE from
.env.example. Option A from the issue.

Merge-gate evidence: negative claim independently re-established (no Go, shell,
Dockerfile, k8s, CI or infra-repo caller); task check exit 0 across both
modules; CI run 7374 green on the PR head.

Co-authored-by: Mathias <mthbqv@gmail.com>

.env.example

@@ -1,7 +1,6 @@
 # Supervisor MCP server
 SUPERVISOR_PORT=3200
 SUPERVISOR_CONFIG_DIR=./config/supervisor
-SUPERVISOR_MODELS_FILE=./config/models.yaml
 
 # LiteLLM gateway (iguana)
 LITELLM_BASE_URL=http://iguana:4000