refactor(gateway): share agent prompt builder

src/gateway/agent-prompt.ts

@@ -0,0 +1,43 @@
+import { buildHistoryContextFromEntries, type HistoryEntry } from "../auto-reply/reply/history.js";
+
+export type ConversationEntry = {
+  role: "user" | "assistant" | "tool";
+  entry: HistoryEntry;
+};
+
+export function buildAgentMessageFromConversationEntries(entries: ConversationEntry[]): string {
+  if (entries.length === 0) {
+    return "";
+  }
+
+  // Prefer the last user/tool entry as "current message" so the agent responds to
+  // the latest user input or tool output, not the assistant's previous message.
+  let currentIndex = -1;
+  for (let i = entries.length - 1; i >= 0; i -= 1) {
+    const role = entries[i]?.role;
+    if (role === "user" || role === "tool") {
+      currentIndex = i;
+      break;
+    }
+  }
+  if (currentIndex < 0) {
+    currentIndex = entries.length - 1;
+  }
+
+  const currentEntry = entries[currentIndex]?.entry;
+  if (!currentEntry) {
+    return "";
+  }
+
+  const historyEntries = entries.slice(0, currentIndex).map((e) => e.entry);
+  if (historyEntries.length === 0) {
+    return currentEntry.body;
+  }
+
+  const formatEntry = (entry: HistoryEntry) => `${entry.sender}: ${entry.body}`;
+  return buildHistoryContextFromEntries({
+    entries: [...historyEntries, currentEntry],
+    currentMessage: formatEntry(currentEntry),
+    formatEntry,
+  });
+}