feat(typefully): support X subscribers-only drafts

[linuz90]

TL;DR

Adds Typefully skill and CLI support for X Subscribers-only draft posts. Agents can now mark every X post or exact post indexes as Subscribers-only, with docs making the Creator Subscriptions eligibility limits clear.

Summary

• Add Subscribers-only draft flags with --subscribers-only for all X posts and --subscribers-only-posts <all|none|0,2> for exact zero-based per-post selection, mapping directly to platforms.x.posts[n].subscribers_only.
• Keep the option X-only and audience-safe by applying the flag only to X posts in multi-platform drafts, accepting snake_case aliases quietly, validating out-of-range post indexes, and rejecting X Community combinations before calling the API.
• Support create/update aliases and metadata-only updates so create-draft and update-draft stay feature-equivalent, and existing X drafts can have subscribers-only flags set or cleared without rewriting text.
• Document eligibility and caveats in the skill docs, CLI help, and changelog, including that successful publishing requires an eligible, approved X Creator Subscriptions account with an active offering.

Validation

• node --check skills/typefully/scripts/typefully.js
• npm run test
• git diff --check

Manual QA

• Create an X draft with --subscribers-only → every X post in the API payload is marked subscribers_only: true.
• Create a mixed X thread with --subscribers-only-posts 1 → only the second X post is marked Subscribers-only.
• Run drafts:update <draft_id> --subscribers-only-posts none on an existing X draft → existing X post text is preserved and subscribers-only flags are cleared.
• Try --subscribers-only with --platform linkedin or --community → the CLI returns a clear local validation error before sending a request.

After Merge

No action required

Related Links

• Backend PR: typefully/typefully-backend#1639
• X Creator Subscriptions requirements
• X Creator Monetization Standards