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>

config/models.yaml

@@ -1,26 +0,0 @@
-# Model selection — first entry per skill is used.
-# Override per-call by passing model in the MCP tool args.
-# Model names come from LiteLLM /v1/models (host/name format).
-
-default_chain:
-  - iguana/qwen3-coder-next
-
-skills:
-  tdd:
-    chain:
-      - koala/qwen3-coder-30b
-  review:
-    chain:
-      - iguana/devstral
-  debug:
-    chain:
-      - iguana/deepseek-r1-14b
-  spec:
-    chain:
-      - koala/phi4-14b
-  retrospective:
-    chain:
-      - iguana/qwen3-coder-next
-  trainer:
-    chain:
-      - iguana/qwen3-coder-next