ops: enable GitHub Discussions + create `channel-ideas` category

[distroinfinity]

Context

The M8 landing footer has a "Submit a channel" link pointing to:

```
https://github.com/distroinfinity/devdrip/discussions/categories/channel-ideas
```

Currently 404s because Discussions isn't enabled on the repo. Defined in `frontend/components/landing/coming-channels-card.tsx`.

Steps

1. Repo Settings → Features → enable Discussions
2. Settings → Discussions → New category:
   • Name: `Channel ideas`
   • Slug: `channel-ideas` (must match — the landing link hardcodes this slug)
   • Format: Open-ended discussion (or Ideas, both fine)
   • Description: e.g. "Pitch a new Distro TV channel — weather, build status, sports, calendar, deploy logs, whatever fits the ambient slot."

Acceptance

• Footer "Submit a channel" link resolves to the new category instead of 404

Related

• M8 PR: M8: landing page + install vector #70
• Component: `frontend/components/landing/coming-channels-card.tsx`