docs+test: the young-run window in SERVER-MODE/LANES; run.test plants its link in the real drop dir

run.test.js hardcoded /tmp/agenthub-drops-<uid>, which drops left for
<data dir>/drops long ago — it passed only while an old build's folder
survived in /tmp (gone after a reboot). Full suites: server 2887/2887,
client 1250/1250.

server/__tests__/run.test.js

@@ -335,17 +335,15 @@ describe("GET /api/run/drop/:name", () => {
   });
 
   it("serves no file that is not a dropped image or text file", async () => {
-    const link = path.join(
-      os.tmpdir(),
-      `agenthub-drops-${os.userInfo().uid}`,
-      "fedcba9876543210.png"
-    );
-    fs.rmSync(link, { force: true });
-    fs.symlinkSync("/etc/passwd", link);
     const pdf = await fetchJson("/api/run/drop-file?name=a.pdf", {
       method: "POST",
       raw: { type: "application/pdf", buf: Buffer.from("%PDF") },
     });
+    // Planted in the real drop dir (the one this drop just made), not a
+    // guessed path: `<data dir>/drops` replaced `/tmp/agenthub-drops-<uid>`.
+    const link = path.join(path.dirname(pdf.body.path), "fedcba9876543210.png");
+    fs.rmSync(link, { force: true });
+    fs.symlinkSync("/etc/passwd", link);
     for (const name of [
       path.basename(pdf.body.path), // a drop, but no image
       "0123456789abcdef.png", // the right shape, never dropped