Print resume instructions on the way out

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/farewell.ts

@@ -0,0 +1,35 @@
+/** What the session was, for the line printed as shiro exits. */
+export type Farewell = {
+  id: string;
+  messages: number;
+  title: string;
+};
+
+/** Long enough to be unique in practice, short enough to retype from the screen. */
+const PREFIX = 8;
+
+const clip = (s: string, n: number) => (s.length > n ? `${s.slice(0, n - 3)}...` : s);
+
+/**
+ * The exit message: good bye, and how to pick this session up again.
+ *
+ * A session's id is a UUIDv7 nobody retypes, so the resume line shows the prefix
+ * `resolveId` accepts alongside `-c`, which needs no id at all. An empty session was
+ * never persisted, so it gets no resume command — pointing someone at `-c` that finds
+ * nothing is worse than saying nothing.
+ */
+export function farewell({ id, messages, title }: Farewell): string {
+  if (messages === 0) return 'Good bye. Nothing to save from this session.';
+
+  const count = `${messages} message${messages === 1 ? '' : 's'}`;
+  const named = title && title !== 'untitled' ? `: "${clip(title, 52)}"` : '';
+
+  return [
+    'Good bye.',
+    `Saved ${count}${named}`,
+    '',
+    'Resume it with:',
+    `  shiro -c                    newest session in this directory`,
+    `  shiro -r ${id.slice(0, PREFIX)}           this session by id`,
+  ].join('\n');
+}