Add implement-code and plan-design skills

[ChrisJBurns]

Summary

Adds two Claude Code skills that were previously only available locally (blocked by global ~/.gitignore). These skills provide structured workflows for feature design (/plan-design) and implementation (/implement-code) using subagents, TDD, and verification gates.

Type of change

• Other (tooling/config)

Test plan

• Verified skill files have valid frontmatter
• Confirmed skills appear in the available skills list

Large PR Justification

• have added 2 skills, can separate if folks want

🤖 Generated with Claude Code

[github-actions]

Large PR Detected

This PR exceeds 1000 lines of changes and requires justification before it can be reviewed.

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## Large PR Justification

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.06%. Comparing base (afce182) to head (9ff9daf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4854   +/-   ##
=======================================
  Coverage   69.06%   69.06%           
=======================================
  Files         530      530           
  Lines       55127    55127           
=======================================
  Hits        38071    38071           
  Misses      14128    14128           
  Partials     2928     2928           

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Large PR justification has been provided. Thank you!

[github-actions]

✅ Large PR justification has been provided. The size review has been dismissed and this PR can now proceed with normal review.

[ChrisJBurns]

Closing, put it in stacklok plugins instead as its not toolhive specific