Add GitHub pull request publishing

[bnz183]

Summary

• Adds direct, pull-request, and draft-pull-request publishing modes for protected GitHub repositories.
• Preserves existing direct commit publishing for GitHub and other publishers (WordPress, Ghost, GitLab, Bitbucket).
• Adds Studio publish mode selector with PR URL feedback and demo-mode simulation.

Test plan

• pnpm build
• pnpm test
• pnpm test:e2e
• Verify direct publish still works against an unprotected repo
• Verify PR publish opens/updates a sourcedraft/{slug} branch and PR on a protected repo
• Verify SOURCEDRAFT_PUBLISH_MODE=pull-request server default is reflected in Studio config

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