Discord plugin: support category-level access config

Feature Request

Plugin: Discord channel plugin (discord / claude-plugins-official)

Current behavior

The groups map in ~/.claude/channels/discord/access.json only supports individual channel IDs:

"groups": {
  "<channelId>": { "requireMention": false, "allowFrom": [] }
}

To allow the bot in multiple channels, each must be added one by one via /discord:access group add <channelId>.

Requested behavior

Support Discord category IDs in the groups map, so all channels under a category are covered automatically:

"groups": {
  "<categoryId>": { "requireMention": false, "allowFrom": [], "isCategory": true }
}

The channel server would check if an incoming message's channel has a parent_id matching any configured category ID, and apply that category's rules.

Why

When you want the bot available across an entire section of your server (e.g. a "Claude" category with several channels), adding each channel manually is tedious and breaks whenever new channels are added under that category.

Suggested implementation

• Add optional "isCategory": true flag to group entries (or auto-detect via Discord API — category channels have type: 4)
• On inbound message, check channel.parent_id against the groups map
• Fall back to exact channel ID match if no category match found