expand limit when running subprocess

[touchaponk]

This pull request introduces a minor enhancement to the start method in src/dolphin_mcp/client.py. The change increases the buffer size for stdout to improve handling of larger outputs.

This is beneficial because the current buffer size is too small for some use cases, causing the subprocess to hang when the buffer is full.

• src/dolphin_mcp/client.py: Added a limit parameter to the asyncio.create_subprocess_exec call, setting a larger buffer size for stdout (1 << 20).