fix(spaceshd): isolate instance-lock test via SPACESH_LOCK

lock_is_exclusive_within_process acquired the global ~/.spacesh/daemon.lock,
so it flaked whenever a real daemon was running. Add a SPACESH_LOCK env
override to lock_path() and point the test at a private temp file under a
serial() guard, making the suite deterministic regardless of a live daemon.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/src/lifecycle.rs

@@ -19,6 +19,11 @@ pub fn socket_path() -> Result<PathBuf> {
 }
 
 pub fn lock_path() -> Result<PathBuf> {
+    if let Ok(p) = std::env::var("SPACESH_LOCK") {
+        if !p.is_empty() {
+            return Ok(PathBuf::from(p));
+        }
+    }
     Ok(spacesh_dir()?.join("daemon.lock"))
 }
 
@@ -64,6 +69,11 @@ mod tests {
 
     #[test]
     fn lock_is_exclusive_within_process() {
+        let _serial = crate::test_support::serial();
+        // Use a private lock path so a real running daemon (which holds the
+        // global ~/.spacesh/daemon.lock) can't make this test flake.
+        let tmp = std::env::temp_dir().join("spacesh-lock-exclusive-test.lock");
+        std::env::set_var("SPACESH_LOCK", &tmp);
         let first = acquire_instance_lock().unwrap();
         assert!(first.is_some(), "first acquire should succeed");
         // A second attempt from the same process on the same fd path:
@@ -72,6 +82,8 @@ mod tests {
         let second = acquire_instance_lock().unwrap();
         assert!(second.is_none(), "second acquire should be blocked");
         drop(first);
+        std::env::remove_var("SPACESH_LOCK");
+        let _ = std::fs::remove_file(&tmp);
     }
 
     #[test]