feat: restructure marketplace for individual plugin selection

BREAKING CHANGE: Each skill, agent, and command is now an individual plugin

- Rename marketplace from skills-collection to agent-skills
- Add 51 individual plugin entries (39 skills, 6 agents, 6 commands)
- Each plugin uses strict: false with explicit paths
- Remove plugin.json (redundant with new marketplace structure)
- Update README with new installation instructions
- Update add-skill command to add marketplace.json entries
- Update bump_version.py to only use marketplace.json

Author: leonardocouy