fix(browser): claim the Chrome-fallback socket dir before using it

_run_chrome_fallback_command creates agent-browser-<session> in the shared
tmpdir and then opens stdout/stderr files inside it, but never writes the
<session>.owner_pid marker. _reap_orphaned_browser_sessions rmtree's any
agent-browser-* dir that carries no live owner and is not tracked in the
calling process, so a second hermes process — or a parallel test worker —
deletes the directory between the makedirs and the first os.open, and the
command dies with FileNotFoundError on _stdout_open.

Write the owner marker immediately after creating the directory, which is
what every other socket-dir user already does.

Deterministic repro on main: run any test that exercises the fallback while
a second process calls _reap_orphaned_browser_sessions() in a loop —
5/5 fail before, 6/6 pass after.

tools/browser_tool.py

@@ -1357,6 +1357,10 @@ def _run_chrome_fallback_command(
 
     task_socket_dir = os.path.join(_socket_safe_tmpdir(), f"agent-browser-{tmp_session}")
     os.makedirs(task_socket_dir, mode=0o700, exist_ok=True)
+    # Claim the dir before using it: another hermes process's orphan reaper
+    # rmtree's any agent-browser-* dir in the shared tmpdir that carries no
+    # live owner, which otherwise deletes this one mid-command.
+    _write_owner_pid(task_socket_dir, tmp_session)
     browser_env = _build_browser_env()
     browser_env["AGENT_BROWSER_SOCKET_DIR"] = task_socket_dir
     browser_env["PATH"] = _merge_browser_path(browser_env.get("PATH", ""))