feat: parallel tool execution with automatic mutation serialization

- Add lightweight async mutex (src/util/mutex.ts) for FIFO serialization
- Add serialized?: boolean to BrowserToolDefinition
- Wire mutex acquisition into registerBrowserTool execute wrapper
- Expose mutationMutex() on BrowserClient
- Mark all mutation tools (click, type, scroll, navigate, switch_tab, etc.) as serialized
- Leave observation tools (screenshot, page_info, execute_js, http_get, etc.) unguarded
- Update prompt and skill docs to reflect parallel-safe behavior
- Fix scroll tool: add Page.bringToFront, explicit button:none/buttons:0/pointerType:mouse
- Update .gitignore to ignore scripts/ and data/ folders

1 files changed, 25 insertions, 0 deletions

diff --git a/src/util/mutex.ts b/src/util/mutex.ts
new file mode 100644
index 0000000..08efe76
--- /dev/null
+++ b/src/util/mutex.ts
@@ -0,0 +1,25 @@
+/**
+ * Lightweight async mutex (~10 lines). Acquire returns a release function.
+ * When the mutex is free, acquire() resolves immediately. When held,
+ * callers are queued and execute serially in FIFO order.
+ */
+export type Mutex = {
+  acquire(): Promise<() => void>;
+};
+
+export const createMutex = (): Mutex => {
+  let queue = Promise.resolve<unknown>(undefined);
+
+  return {
+    async acquire(): Promise<() => void> {
+      let release = () => {};
+      const next = new Promise<void>((resolve) => {
+        release = resolve;
+      });
+      const prev = queue;
+      queue = prev.then(() => next);
+      await prev;
+      return release;
+    },
+  };
+};