feat: add last message preview to session picker

Read the final user/assistant message from session transcripts and display
it in the picker alongside the session update time. Allows quick previews
of what's in each session without opening it.

src/gateway/session-utils.fs.ts

@@ -137,3 +137,53 @@ export function readFirstUserMessageFromTranscript(
   }
   return null;
 }
+
+const LAST_MSG_MAX_BYTES = 16384;
+const LAST_MSG_MAX_LINES = 20;
+
+export function readLastMessagePreviewFromTranscript(
+  sessionId: string,
+  storePath: string | undefined,
+  sessionFile?: string,
+  agentId?: string,
+): string | null {
+  const candidates = resolveSessionTranscriptCandidates(sessionId, storePath, sessionFile, agentId);
+  const filePath = candidates.find((p) => fs.existsSync(p));
+  if (!filePath) return null;
+
+  let fd: number | null = null;
+  try {
+    fd = fs.openSync(filePath, "r");
+    const stat = fs.fstatSync(fd);
+    const size = stat.size;
+    if (size === 0) return null;
+
+    const readStart = Math.max(0, size - LAST_MSG_MAX_BYTES);
+    const readLen = Math.min(size, LAST_MSG_MAX_BYTES);
+    const buf = Buffer.alloc(readLen);
+    fs.readSync(fd, buf, 0, readLen, readStart);
+
+    const chunk = buf.toString("utf-8");
+    const lines = chunk.split(/\r?\n/).filter((l) => l.trim());
+    const tailLines = lines.slice(-LAST_MSG_MAX_LINES);
+
+    for (let i = tailLines.length - 1; i >= 0; i--) {
+      const line = tailLines[i];
+      try {
+        const parsed = JSON.parse(line);
+        const msg = parsed?.message as TranscriptMessage | undefined;
+        if (msg?.role === "user" || msg?.role === "assistant") {
+          const text = extractTextFromContent(msg.content);
+          if (text) return text;
+        }
+      } catch {
+        // skip malformed
+      }
+    }
+  } catch {
+    // file error
+  } finally {
+    if (fd !== null) fs.closeSync(fd);
+  }
+  return null;
+}