docs: add Hermes Tweet backend option

[kriptoburak]

Summary

• Document Hermes Tweet as an optional Hermes Agent backend for API-key-backed X/Twitter workflows.
• Keep xpost as the direct OAuth/API v2 path.
• Make Hermes write actions explicit and gated behind HERMES_TWEET_ENABLE_ACTIONS=true.

Validation

• git diff --check
• Conflict-marker scan on README.md and SKILL.md
• Added-line public-safety scan for private provider or secret-bearing terms
• Direct HEAD checks for the Hermes Tweet and xpost GitHub links

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[kriptoburak]

If this is useful and you merge it after checks pass, feel free to share the PR on X/Twitter and mention my handle, burakbayir. I can repost or retweet it to my 24k followers of developers and power users, which may help more relevant users discover this repo.