feat(ai): single point of provider selection

src/lib/ai/index.ts

@@ -0,0 +1,41 @@
+import "server-only";
+
+import { loadAiSettings } from "@/lib/ai/settings";
+import { buildSystemPrompt } from "@/lib/ai/prompt";
+import { createAnthropicProvider } from "@/lib/ai/provider-anthropic";
+import { createOpenAiCompatibleProvider } from "@/lib/ai/provider-openai";
+import { AiError, type AiProvider } from "@/lib/ai/types";
+
+/**
+ * The only module that decides which provider is active.
+ *
+ * Every consumer asks for `getProvider()` and gets the interface. Nothing else
+ * in the codebase should import an adapter directly — that is what keeps
+ * switching provider a settings change.
+ */
+export async function getProvider(): Promise<AiProvider> {
+  const settings = await loadAiSettings();
+
+  if (!settings?.apiKey) {
+    throw new AiError("No AI provider is configured", false);
+  }
+
+  if (settings.provider === "anthropic") {
+    return createAnthropicProvider(settings.apiKey, settings.model);
+  }
+
+  return createOpenAiCompatibleProvider(
+    settings.apiKey,
+    settings.model,
+    settings.provider
+  );
+}
+
+/** The full system prompt for the current settings. */
+export async function getSystemPrompt(): Promise<string> {
+  const settings = await loadAiSettings();
+  return buildSystemPrompt(settings?.systemPromptExtra ?? "");
+}
+
+export { AiError };
+export type { AiProvider, CompletionRequest, CompletionResult } from "@/lib/ai/types";