feat: package Claude Code plugin

[cablate]

Summary

• add Claude Code plugin and marketplace manifests backed by the existing three Skills
• package a versioned plugin ZIP and SHA-256 checksum in CI and releases
• validate the plugin with Claude Code's official strict validator on Linux
• document marketplace installation in English and Traditional Chinese

Verification

• npm run lint (0 errors; existing warnings remain)
• npm run test:unit (37 passed)
• npm test (114 passed)
• npm run build
• npm run package:claude-plugin
• git diff --check

Local Windows cannot execute the bundled Claude CLI binary, so the official strict validation is intentionally enforced by the Ubuntu CI job.