Add Hermes Tweet plugin example

[kriptoburak]

Summary

• Add an optional plugins section for Fly.io-hosted Hermes deployments.
• Use Hermes Tweet as the concrete example because it needs both plugin installation and a Fly secret, which shows the deployment pattern clearly.
• Keep write-capable X/Twitter actions explicitly gated by default.

Duplication checks

• Searched repo contents for hermes-tweet, Hermes Tweet, Xquik, xquik, tweet, Twitter, and X/Twitter before editing.
• Checked open and closed issues and PRs for existing Hermes Tweet submissions.
• Found no existing Hermes Tweet listing or proposal.

Verification

• git diff --check