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

Closes #116.

`config/models.yaml` declared a default chain and six per-skill chains. The
issue filed against it said the deployed service does not read the file. That
was true and understated: **nothing reads it at all**, and the loader written to
read it is dead too.

The whole chain goes:

- `config/models.yaml` — data with no consumer
- `internal/config/models.go` — `LoadModels` and `ModelFor`, whose only callers
  are in `models_test.go`
- `internal/config/config.go` — the `ModelsFile` field, assigned from
  `SUPERVISOR_MODELS_FILE` and never read
- `.env.example` — the variable that configures the path to the file nobody opens

Deleting the YAML alone would have left a loader pointing at a missing file:
one decoy replaced by another.

This subsystem had THREE PASSING TESTS. That is what kept it invisible. Green
assertions proving a function nobody calls works correctly are worse than no
tests, because coverage is exactly the signal someone would use to decide the
code is alive. The repo's own
`docs/superpowers/specs/2026-04-20-model-orchestration-design.md` said so in
April — "the model routing config exists but is dead weight" — and it survived
five more months anyway.

Two of the six models it named no longer exist (`iguana/qwen3-coder-next`,
retired in infra#416, and `koala/qwen3-coder-30b`, gone earlier). Nobody noticed
because the file being unread is the only reason it never failed.

Live routing is env, on the deployed `ingestion` service: `INGEST_LLM_MODEL`,
`BRAIN_LLM_PRIMARY_MODEL`, and four more. Not one name from the deleted file
appears there. Env is the real configuration surface; this file read as though
it were, which is why editing it to change routing would have changed nothing,
silently.

`go build ./...` clean, `task check` EXIT=0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -7,7 +7,6 @@ type Config struct {
 	LiteLLMBaseURL string // LITELLM_BASE_URL, default http://iguana:4000
 	LiteLLMAPIKey  string // LITELLM_API_KEY
 	ConfigDir      string // SUPERVISOR_CONFIG_DIR, default ./config/supervisor
-	ModelsFile     string // SUPERVISOR_MODELS_FILE, default <ConfigDir>/../models.yaml
 	IngestBaseURL  string // INGEST_BASE_URL, default http://localhost:3300
 	IngestSvcURL   string // INGEST_SVC_URL — base URL for brain_ingest (/ingest, /ingest-path)
 	KBRetrievalURL string // KB_RETRIEVAL_URL — base URL for brain_search
@@ -23,7 +22,6 @@ func Load() (Config, error) {
 		LiteLLMAPIKey:  os.Getenv("LITELLM_API_KEY"),
 		ConfigDir:      envOr("SUPERVISOR_CONFIG_DIR", "./config/supervisor"),
 	}
-	cfg.ModelsFile = envOr("SUPERVISOR_MODELS_FILE", cfg.ConfigDir+"/../models.yaml")
 	cfg.IngestBaseURL = envOr("INGEST_BASE_URL", "http://localhost:3300")
 	cfg.IngestSvcURL = envOr("INGEST_SVC_URL", "")
 	cfg.KBRetrievalURL = envOr("KB_RETRIEVAL_URL", "")