[Bug] Setting SLACK_MCP_ADD_MESSAGE_TOOL environment variable to comma separated string still allows messages to be sent to all channels

Hi, thanks for your work here!

I tested `SLACK_MCP_ADD_MESSAGE_TOOL` with my own DM channel ID, and a comma separated list of whitelisted DM channel IDs, and I was still able to send messages to other channels and people not in that list.

Claude suggested there may be an issue with the negation logic in `isChannelAllowed()`.

Thanks!