SKILLS_DIR was hardcoded to the dev monorepo path which doesn't exist
when mmagent is installed globally via npm. Now tries the dev path
first, falls back to resolving @zhixuan92/multi-model-agent-core's
src/skills/ via createRequire. Also adds src/skills to core's files
field so skill markdown is always shipped.

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