fix: v2.7.3 — Update now restarts through our own service unit, member panes stay live on a dev server, a start refuses a missing folder

- self-restart: a server in our unit's cgroup (/agenthub.service, system
  or user) restarts via `systemctl restart --no-block` (sudo -n for the
  system unit), our launchd job by exiting into KeepAlive. The detached
  restart-helper replacement held the port while Restart=always started
  the unit every 3 s (v2.7.2 on the S3 box: 240+ restarts, MainPID 0).
- run-as.mayHaveTmux: only ENOENT means "no tmux". A dev server in server
  mode without CAP_DAC_READ_SEARCH got EACCES on a member's 0700
  .agenthub/tmux, dropped the member from the list and ended their runs
  within seconds — the real cause of `401 not a live pane's token`.
- tmux: a member's list is re-asked every 5 s, not 1 s (one sudo + three
  auth.log lines each).
- POST /api/lanes/:id/start: 409 ENOCWD for a folder that no longer
  exists (the pane started in the account's home otherwise).
- Settings: Remote sources only for staff (a developer got a 403).
- scripts/test-server.js drops every inherited AGENTHUB_* (pane env).
Tests: self-restart.test.js, server-s3-panes (mayHaveTmux), lanes-api
(ENOCWD); touched suites 771 + client pages 191, tsc -b.

server/__tests__/server-s3-panes.test.js

@@ -248,4 +248,20 @@ describe("run-as: a path under the users root belongs to that user", () => {
       "/opt/agenthub/libexec/write-as"
     );
   });
+  it("counts a member's tmux it cannot stat as maybe there, a missing one as none", () => {
+    const home = path.join(TMP, "users", "zoe");
+    const acct = { name: "zoe", login: "ah-zoe", home, uid: 5002 };
+    assert.equal(runAs.mayHaveTmux(acct), false, "no .agenthub at all");
+    const dir = path.join(home, ".agenthub", "tmux");
+    fs.mkdirSync(dir, { recursive: true });
+    assert.equal(runAs.mayHaveTmux(acct), false, "no socket yet");
+    // The member's own 0700 dir, seen by a dashboard without CAP_DAC_READ_SEARCH.
+    fs.chmodSync(dir, 0o000);
+    try {
+      if (process.getuid() !== 0)
+        assert.equal(runAs.mayHaveTmux(acct), true, "EACCES proves nothing");
+    } finally {
+      fs.chmodSync(dir, 0o700);
+    }
+  });
 });