feat: add zyfai-yield skill — CLI-driven DeFi yield via Zyfai SDK

[tonyagents]

Supersedes the pending fixes from PR #12 which the original contributor hasn't applied.

Changes

• SKILL.md fully rewritten — no inline TypeScript. All SDK operations called via node scripts/zyfai-*.js (shell-executable)
• 3 helper scripts added: scripts/zyfai-deploy.js, zyfai-deposit.js, zyfai-withdraw.js — wraps @zyfai/sdk with clean CLI interfaces
• zyfai-yield moved to own zyfai-skills plugin block in marketplace.json
• Added Error Handling section and expanded tags
• PR title corrected to feat: as requested

All verified items from Kevin's review preserved: @zyfai/sdk real, API endpoint real, cross-references valid.

[kevarifin14]

A+ — Ready to merge

Complete rewrite addressing all D+ blockers from #12:

• SKILL.md now CLI-only (calls node ~/.zyfai-scripts/scripts/zyfai-*.js)
• Own zyfai-skills plugin block
• Correct feat: title
• Env vars ($WALLET_PRIVATE_KEY, $ZYFAI_API_KEY) instead of embedded secrets
• Error handling section added

Verified: @zyfai/sdk real npm, API endpoint live, cross-refs resolve.

Merging. Closing #12 as superseded.

[kevarifin14]

Superseded by #61 (cherry-pick onto current main, marketplace.json conflict resolved). Closing. Credit preserved via Co-Authored-By. Thanks @tonyagents!

[kevarifin14]

Reopening was overridden — superseded cherry-pick #61 also had to be closed. Issue: scripts/*.js violates the repo's no-code rule (per CONTRIBUTING.md: "No Python scripts, no TypeScript SDK code. All execution happens through CLIs"). A path forward would be publishing a real zyfai-cli npm package with a proper binary, then rewriting the skill to call that CLI. Helper scripts shipped inside the skill directory don't meet the bar.