fix(mem0-plugin): remove userConfig from plugin.json, add auto-deps install

- Remove userConfig from .claude-plugin/plugin.json — OAuth flow handles
  auth via mcp.mem0.ai, userConfig prompt was redundant and confusing
- Add ensure_deps.sh to install mem0ai SDK into persistent plugin venv
- Add requirements.txt (mem0ai) for dependency tracking
- Wire SessionStart hook to auto-install deps on first run
- Update setup_coding_categories.py to find venv site-packages
- Update onboarding SKILL.md with venv-aware install steps

mem0-plugin/requirements.txt

@@ -0,0 +1 @@
+mem0ai