refactor: unify the two Anthropic-likeness classifiers (hoist vs cache-control)

Found during v0.8.3 release review (CTO persona). **Deferred** because it touches a hot path and changes cache-control behavior for gateway traffic — needs design + careful testing (>30 min), not for a patch release.

## Context
There are now two independent predicates deciding whether a model is "Anthropic-like":
1. `isAnthropicLikeModel` in `packages/opencode/src/session/prompt.ts` — gates the `` system-role **hoist** (added in #888). Built on `familyVendor` + providerID + api.id/npm heuristics.
2. The inline cache-control check in `packages/opencode/src/provider/transform.ts:284-295` — gates Anthropic `cache_control` injection. Hard-codes `model.providerID === "altimate-backend"` as Anthropic-for-caching.

These **deliberately diverge today**: `transform.ts` treats the `altimate-backend` gateway as Anthropic-for-caching, while `isAnthropicLikeModel` classifies the same gateway (`family: openai`) as non-Anthropic. The divergence is correct for the #888 fix, but the two classifiers will drift further apart as gateway models expand, and a future maintainer could unify them wrongly.

## Suggested fix
Extract a single shared predicate (built on `familyVendor`) used by both the hoist decision and the cache-control decision, with explicit handling of the gateway case so the two behaviors can't silently drift. Verify cache-hit rates on gateway traffic don't regress.

## Scope
`packages/opencode/src/session/prompt.ts`, `packages/opencode/src/provider/transform.ts`, `packages/opencode/src/provider/family.ts`.