refactor(agents): extract shared session dir resolver

src/agents/session-dirs.ts

@@ -0,0 +1,22 @@
+import type { Dirent } from "node:fs";
+import fs from "node:fs/promises";
+import path from "node:path";
+
+export async function resolveAgentSessionDirs(stateDir: string): Promise<string[]> {
+  const agentsDir = path.join(stateDir, "agents");
+  let entries: Dirent[] = [];
+  try {
+    entries = await fs.readdir(agentsDir, { withFileTypes: true });
+  } catch (err) {
+    const code = (err as { code?: string }).code;
+    if (code === "ENOENT") {
+      return [];
+    }
+    throw err;
+  }
+
+  return entries
+    .filter((entry) => entry.isDirectory())
+    .map((entry) => path.join(agentsDir, entry.name, "sessions"))
+    .toSorted((a, b) => a.localeCompare(b));
+}