Add vault X bookmarks capture skill

[markphelps]

Summary

• Add a vault-x-bookmarks skill with a TypeScript helper that captures bounded X bookmark slices into raw/sources and records durable state under raw/state/x-bookmarks.
• Use a skill-local package with dotenv and xurl as the external authenticated transport, including long-post note_tweet handling.
• Document setup/run requirements and sync the generated top-level skill mirror.

Test Plan

• npm run test:x-bookmarks
• npx tsc --noEmit --module NodeNext --moduleResolution NodeNext --target ES2022 --types node scripts/x-bookmarks.ts scripts/x-bookmarks.test.ts
• npm run validate
• npm run format:check