Discord channel binding generator (bind-discord.sh)

[dahifi]

The Discord openclaw.json channel binding config is the most error-prone manual step in deployment.

New script: scripts/bind-discord.sh <bot-token> that:

1. Calls Discord API to fetch server list + channels
2. Presents interactive selection or dumps all
3. Generates the channel binding JSON block for openclaw.template.json

Eliminates hand-editing JSON channel IDs.

Implementation notes: no separate validation report docs in PR, keep docs minimal.