docs: add Hermes Tweet spec context

[kriptoburak]

Summary

• Add a compact Companion X/Twitter Specs section.
• Explain how Hermes Tweet monitoring context can pair with OpenSpec proposals, tasks, criteria, and spec diffs.
• Keep disclosure explicit that Hermes Tweet is third-party.

Eligibility

• Target repo is an active Hermes Agent OpenSpec plugin.
• License reviewed: MIT.
• Duplicate search found no existing Hermes Tweet PR in this repository.

Validation

• git diff --check
• Added-line safety scan for broken-link, secret, and forbidden-text patterns