Add GitHub Discussions integration

[SamyPesse]

Summary

• add new integration github-conversations
• allow OAuth with GitHub
• ingest closed discussions from selected repo
• ingest discussion when webhook event fires
• remove the icon until artwork is finalized

Testing

• bun run typecheck (fails: cannot find modules for @gitbook/api)
• bun run test (fails: cannot find modules for @gitbook/api)

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https://chatgpt.com/codex/tasks/task_b_6851ade595c8833381a081e59b88c25d

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Latest commit: 01df1f1

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