feat(prompts-core): add variant resolver

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

packages/prompts-core/src/variant-resolver.ts

@@ -0,0 +1,59 @@
+import {
+  isClaudeOpus47Model,
+  isGeminiModel,
+  isGlmModel,
+  isGptModel,
+  isKimiK2Model,
+  isMiniMaxModel,
+} from "@oh-my-opencode/model-core"
+import type { VariantTable } from "./types"
+
+export type ResolveVariantInput = {
+  readonly modelID?: string
+  readonly agentName?: string
+  readonly variants: VariantTable
+}
+
+const PLANNER_AGENT_NAMES = new Set(["prometheus"])
+
+const MODEL_MATCHERS: Record<string, (modelID: string) => boolean> = {
+  gpt: isGptModel,
+  gemini: isGeminiModel,
+  kimi: isKimiK2Model,
+  glm: isGlmModel,
+  "opus-4-7": isClaudeOpus47Model,
+  minimax: isMiniMaxModel,
+}
+
+export function resolveVariant(input: ResolveVariantInput): string {
+  const variantNames = Object.keys(input.variants)
+  if (variantNames.length === 0) {
+    throw new TypeError("resolveVariant requires at least one prompt variant")
+  }
+
+  if (isPlannerAgent(input.agentName) && variantNames.includes("planner")) {
+    return "planner"
+  }
+
+  if (input.modelID !== undefined) {
+    for (const variantName of variantNames) {
+      if (matchesModelVariant(variantName, input.modelID)) return variantName
+    }
+  }
+
+  if (variantNames.includes("default")) return "default"
+
+  const firstVariant = variantNames[0]
+  if (firstVariant !== undefined) return firstVariant
+
+  throw new TypeError("resolveVariant requires at least one prompt variant")
+}
+
+function isPlannerAgent(agentName: string | undefined): boolean {
+  return agentName !== undefined && PLANNER_AGENT_NAMES.has(agentName.toLowerCase())
+}
+
+function matchesModelVariant(variantName: string, modelID: string): boolean {
+  const matcher = MODEL_MATCHERS[variantName]
+  return matcher?.(modelID) ?? false
+}