test: red for #258 — temp_cwd sandbox leaks one dir per run into /tmp

The helper keys its sandbox on std::process::id() under env::temp_dir(),
so the pre-create wipe never matches a prior run's path: one directory
strands per test-binary invocation on the 16 GiB tmpfs (>50k dirs by
2026-07-13). The test pins the property at one representative site: the
returned path embeds no PID and lives off env::temp_dir().

refs #258

crates/aura-cli/tests/project_nodes.rs

@@ -18,6 +18,39 @@ fn aura(args: &[&str], cwd: &Path) -> std::process::Output {
         .expect("spawn aura")
 }
 
+/// Property (#258): a test sandbox must not outlive its run — `temp_cwd` must
+/// hand back a run-STABLE path that lives off the `env::temp_dir()` tmpfs, so
+/// its own pre-create `remove_dir_all` reclaims the previous run instead of
+/// stranding one `aura-nodes-*` directory per test-binary invocation.
+///
+/// The helper today keys the name on `std::process::id()`, so every run mints a
+/// new path the wipe can never match: >50 000 leaked dirs filled a 16 GiB tmpfs
+/// to 100%. The in-repo remedy is the knob-lab scaffold's (commit 84e1075, sibling
+/// `project_sweep_campaign.rs`): a fixed name under `CARGO_TARGET_TMPDIR`. This
+/// pins that property at one representative site — the assertion reads the path
+/// value only, so it is deterministic and does not depend on any external state.
+#[test]
+fn temp_cwd_sandbox_does_not_leak_under_env_temp_dir() {
+    let sandbox = temp_cwd("leakguard");
+    // Read the path value, then reclaim immediately so a red run of this very
+    // test does not itself add to the leak it describes.
+    let path = sandbox.clone();
+    let _ = std::fs::remove_dir_all(&sandbox);
+
+    let pid = std::process::id().to_string();
+    assert!(
+        !path.to_string_lossy().contains(&pid),
+        "sandbox name embeds the pid ({pid}), so no later run's pre-create wipe \
+         can reclaim it — one directory leaks per run: {}",
+        path.display(),
+    );
+    assert!(
+        !path.starts_with(std::env::temp_dir()),
+        "sandbox lives under env::temp_dir() — the tmpfs the >50k-dir leak filled: {}",
+        path.display(),
+    );
+}
+
 #[test]
 fn nodes_new_scaffolds_a_sibling_crate_and_attaches_it() {
     let cwd = temp_cwd("attach");