docs: add Sequenzy email marketing skill

[polnikale]

## Summary
- Adds the Sequenzy email marketing Agent Skill to the relevant skill/resource section.
- Keeps the description neutral and aligned with nearby entries.

## Links
- Skill source: https://github.com/Sequenzy/skills/tree/main/skills/sequenzy-email-marketing

• ClawHub: https://clawhub.ai/polnikale/sequenzy-email-marketing

  Verification

  • Reviewed the target README section and contribution format.
  • Verified the Sequenzy skill source is public.
  • Changed files: README.md, skills/sequenzy-email-marketing/SKILL.md, skills/sequenzy-email-marketing/README.md, skills/sequenzy-email-marketing/agents/openai.yaml, skills/sequenzy-email-marketing/references/command-reference.md, skills/sequenzy-email-marketing/references/use-cases.md

  Diff stat:

  README.md | 1 +
  

1 file changed, 1 insertion(+)
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