Follow-up to #136: tests + type/env polish for SH_MODEL_CUSTOM

[moonlight16]

Applies the non-blocking review suggestions from #136 (SH_MODEL_CUSTOM) that should have been included in that PR — they were requested during review but I merged #136 before addressing them. This follow-up closes that gap.

Addresses the four review points from #136:

1. Unit tests (harness/test/run-turn-model.test.ts) — extends the existing model test file:

   • requireModel throws the clear error when SH_MODEL_CUSTOM=1 but ANTHROPIC_BASE_URL is unset.
   • With both set, it synthesizes a model whose id/name = SH_MODEL, baseUrl = ANTHROPIC_BASE_URL, api = anthropic-messages, provider defaults to anthropic, and context/token defaults are applied.
   • SH_MODEL_PROVIDER / SH_MODEL_CONTEXT_WINDOW / SH_MODEL_MAX_TOKENS overrides are honored.

2. Tighten the cast — the synthesized object is now typed Model<"anthropic-messages"> instead of as ReturnType<typeof getModel> & object. If pi-ai’s Model type later gains a required field, this fails to compile instead of silently omitting it.

3. Thread env into requireModel — requireModel(provider, modelId, env = process.env), matching its sibling resolveModelSelection(config, env). This also lets the new tests run hermetically without mutating process.env. The param is optional, so existing 2-arg callers (run-leaf.ts) are unchanged.

4. Env var docs — the four SH_MODEL_CUSTOM / SH_MODEL_CONTEXT_WINDOW / SH_MODEL_MAX_TOKENS / SH_MODEL_PROVIDER vars are documented in README-k8s.md (added in the separate generic-k8s PR Add setup-k8s.sh: generic Kubernetes install path #142), so operators can discover the opt-in.

No behavior change to the default path; purely tests + type-safety + testability.

[mrsabath]

Small, high-quality follow-up to #136 — verified the full diff.

• Type tightening (run-turn.ts): replaces the as ReturnType<typeof getModel> & object cast with a proper Model<"anthropic-messages"> annotation, so tsc now checks the object shape — exactly the "fails to compile instead of silently omitting a field" safety the comment claims. The provider cast is narrowed from as never to Model<...>["provider"].
• Testability: requireModel gains an injectable env param defaulting to process.env — clean and backward-compatible.
• Tests: three assertive tests cover the error path (missing ANTHROPIC_BASE_URL), the synthesis path, and the overrides path (SH_MODEL_PROVIDER / SH_MODEL_CONTEXT_WINDOW / SH_MODEL_MAX_TOKENS). No hidden skips.

Verdict: APPROVE — no issues.

Areas reviewed: TypeScript, tests. Commits: 1, signed-off (DCO green). CI: passing (11/11).