refactor: restructure as marketplace with plugin subdirectory

Moves plugin into plugins/morph-compact/ so the repo can serve as
both a marketplace and a plugin source. Fixes marketplace source
path validation (must be a subdirectory, not ".").

Install flow:
  claude plugin marketplace add morphllm/morph-claude-code-plugin
  claude plugin install morph-compact@morph

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>