Subscription/watch management

[andrew]

The forge APIs support managing notification subscriptions (watch/unwatch repos, subscribe/unsubscribe/ignore threads) but the library doesn't expose this.

• GitHub: PUT /repos/{owner}/{repo}/subscription (watch repo), PUT /notifications/threads/{id}/subscription (subscribe/ignore thread)
• GitLab: subscription endpoints for projects and issues/MRs
• Gitea/Forgejo: PUT /repos/{owner}/{repo}/subscription (watch repo)

This would allow consumers to programmatically mute/unmute notification threads and manage repo watch settings, syncing these preferences back to the forge rather than only tracking them locally.