hive-agent: compact tests set the harness dir like the other Bus tests

The two report_compact tests build a Bus, which resolves the events db under
HYPERHIVE_HARNESS_DIR and panics when it is unset, as in the nix test
sandbox. Add a test-only paths::with_harness_dir that points the variable at
a temp dir under a lock, and use it in both the new tests and the existing
swarm_inference Bus test, which set the variable unguarded.

hive-agent/src/paths.rs

@@ -131,3 +131,38 @@ pub fn claude_dir() -> PathBuf {
     }
     PathBuf::from("/root/.claude")
 }
+
+/// Run `f` with `HYPERHIVE_HARNESS_DIR` pointing at a fresh temp dir, then
+/// restore the previous value. Tests that build a `Bus` or read the harness
+/// state files go through this: the variable is process-wide, so the calls
+/// are serialised on a lock.
+#[cfg(test)]
+pub fn with_harness_dir<T>(f: impl FnOnce() -> T) -> T {
+    use std::ffi::OsString;
+    use std::sync::Mutex;
+
+    static LOCK: Mutex<()> = Mutex::new(());
+
+    struct Restore(Option<OsString>);
+    impl Drop for Restore {
+        fn drop(&mut self) {
+            // SAFETY: still holding `LOCK`, the only writer of this variable.
+            unsafe {
+                match self.0.take() {
+                    Some(v) => std::env::set_var("HYPERHIVE_HARNESS_DIR", v),
+                    None => std::env::remove_var("HYPERHIVE_HARNESS_DIR"),
+                }
+            }
+        }
+    }
+
+    let _lock = LOCK
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner);
+    let dir = tempfile::tempdir().unwrap();
+    // Declared after `_lock` so the variable is restored before the lock drops.
+    let _restore = Restore(std::env::var_os("HYPERHIVE_HARNESS_DIR"));
+    // SAFETY: serialised by `LOCK`.
+    unsafe { std::env::set_var("HYPERHIVE_HARNESS_DIR", dir.path()) };
+    f()
+}