fix: replace shell sleep with cross-platform Atomics.wait in planning lock (#1693)

Fixes #1692

spawnSync('sleep', ['0.1']) fails silently on Windows (ENOENT),
causing a tight busy-loop during lock contention. Atomics.wait()
provides a cross-platform 100ms blocking wait available in Node 22+.

get-shit-done/bin/lib/core.cjs

@@ -581,8 +581,8 @@ function withPlanningLock(cwd, fn) {
           }
         } catch { continue; }
 
-        // Wait and retry
-        spawnSync('sleep', ['0.1'], { stdio: 'ignore' });
+        // Wait and retry (cross-platform, no shell dependency)
+        Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 100);
         continue;
       }
       throw err;