Fix subprocess.run() hanging on commands that read stdin (#281)

Commands like `cat > file.py` (without heredoc) block forever waiting
for stdin input when executed via `execute()`. This happens because
subprocess.run() inherits the parent's stdin by default, which in a
server context is a pipe that never sends data.

Adding stdin=subprocess.DEVNULL gives these commands an immediate EOF,
so they return instantly instead of hanging until the timeout kills them.

This is especially important for agent-based workflows where LLMs may
generate `cat > file` commands expecting interactive input that will
never come.

Author: akhatua2

src/swerex/runtime/local.py

@@ -429,6 +429,7 @@ async def execute(self, command: Command) -> CommandResponse:
                 shell=command.shell,
                 timeout=command.timeout,
                 env=command.env,
+                stdin=subprocess.DEVNULL,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT if command.merge_output_streams else subprocess.PIPE,
                 cwd=command.cwd,