feat(prompts-core): resolve prompt variants by declaration order

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

@@ -1,5 +1,29 @@
-import type { ResolveVariantInput } from "./types"
+import {
+  NoMatchingVariantError,
+  type ResolveVariantInput,
+  type VariantDefinition,
+  type VariantFallback,
+} from "./types"
 
-export function resolveVariant(_input: ResolveVariantInput): string {
-  return "__red__"
+function isVariantFallback(definition: VariantDefinition): definition is VariantFallback {
+  return "fallback" in definition && definition.fallback === true
+}
+
+export function resolveVariant(input: ResolveVariantInput): string {
+  const entries = Object.entries(input.variants)
+  const testInput = {
+    ...(input.modelID === undefined ? {} : { modelID: input.modelID }),
+    ...(input.agentName === undefined ? {} : { agentName: input.agentName }),
+  }
+
+  for (const [variantKey, definition] of entries) {
+    if (isVariantFallback(definition)) continue
+    if (definition.test(testInput)) return variantKey
+  }
+
+  const fallbackEntry = entries.find(([, definition]) => isVariantFallback(definition))
+  if (fallbackEntry !== undefined) return fallbackEntry[0]
+
+  const variantKeys = entries.map(([variantKey]) => variantKey).join(", ")
+  throw new NoMatchingVariantError(`No matching prompt variant found among: ${variantKeys}`)
 }