Add optional Hermes Tweet bookmark backend

[kriptoburak]

Summary

• Add an optional Hermes Tweet/Xquik backend selected with X_SKILL_BACKEND=hermes-tweet.
• Keep the native X API as the default while sharing the existing BookmarksSkill cache and brief pipeline through a small XBookmarkClient contract.
• Add a Hermes Tweet client for bookmark sync, bookmark folders, tweet hydration, account reads, and explicitly opt-in write actions.
• Document setup in .env.example, README, SKILL.md, Claude snippet, and changelog.

Validation

• npm run build
• npm run lint
• npm test -- --runInBand - 6 suites passed, 99 tests passed
• git diff --cached --check
• Staged public text scan for secrets and private operational notes

The integration is intended to be optional and non-breaking: existing X OAuth users keep the current route unless they set the backend environment variable.