feat: initial Lam Agent extension (#1)

src/lib/history.ts

@@ -0,0 +1,30 @@
+import { DEFAULT_SYSTEM_PROMPT, RECENCY_HINT } from "./constants";
+import type { ApiMessage, ChatTurn } from "./types";
+
+/**
+ * Turns stored turns into the message array sent to the model.
+ *
+ * Only completed question/answer pairs are replayed. The intermediate tool
+ * messages from a previous turn are intentionally dropped: they are noise once
+ * the answer is written, and replaying them would re-send large page contents
+ * on every subsequent request.
+ */
+export function buildMessages(systemPrompt: string, turns: ChatTurn[], prompt: string): ApiMessage[] {
+  const messages: ApiMessage[] = [{ role: "system", content: systemPrompt }];
+
+  for (const turn of turns) {
+    if (!turn.response) continue;
+    messages.push({ role: "user", content: turn.prompt });
+    messages.push({ role: "assistant", content: turn.response });
+  }
+
+  messages.push({ role: "user", content: prompt });
+  return messages;
+}
+
+/** Uses the user's prompt if they set one, otherwise Lam's default. */
+export function composeSystemPrompt(customPrompt: string, webEnabled: boolean): string {
+  const parts = [customPrompt.trim() || DEFAULT_SYSTEM_PROMPT];
+  if (webEnabled) parts.push(RECENCY_HINT);
+  return parts.join("\n\n");
+}