repo-settings: enable GitHub Discussions and seed categories

[hummbl-dev]

Summary\nEnable GitHub Discussions for BIF and seed the planned community categories.\n\n## Acceptance criteria\n1. Enable GitHub Discussions in repo settings.\n2. Add categories from docs/DISCUSSIONS.md: Show and tell, Template requests, Q&A, Ideas, Integrations.\n3. Post pinned welcome, share-your-session, and template-request topics from docs/DISCUSSIONS.md.\n\n## Blocker\nRequires repo admin/settings action; cannot be completed as a file-only change.

[hummbl-dev]

Status: Discussions are now enabled and GitHub auto-seeded the default categories (Announcements, General, Ideas, Polls, Q&A, Show and tell). The remaining gap is replacing/adding Template requests and Integrations, which GitHub does not expose through the repo API or gh CLI. I will finish that via the UI/settings path when available.