fix(plugin): remove invalid manifest fields that broke marketplace install

[akiojin]

Summary

• Remove $schema, skills, agents fields from plugin.json that Claude Code's validator rejected, switching to the minimal manifest pattern with directory auto-discovery
• Remove $schema from marketplace.json and sync version to 0.1.0
• Fix agent frontmatter: replace non-standard when-to-use with <example> blocks, hex color with named color, add explicit model
• Add explicit /plugin marketplace add akiojin/unity-cli command and Codex Skills section to all 7 READMEs

Closes #57

Test plan

• python3 -c "import json; json.load(open('plugin.json'))" — valid JSON
• python3 -c "import json; json.load(open('marketplace.json'))" — valid JSON
• skills/ and agents/ directories present for auto-discovery
• Plugin-validator agent passed with no critical issues
• /plugin marketplace add akiojin/unity-cli installs successfully after merge

🤖 Generated with Claude Code

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✨ Finishing Touches

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• Create PR with unit tests
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• Commit unit tests in branch bugfix/issue-57

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