Implements AgentSkillDiscovery class to support discovering Agent Skills from
multiple configured directories across three scopes (global, project, local).
Handles name collisions with precedence: local > project > global.
Adds comprehensive BDD test coverage for multi-scope discovery scenarios including:
- Global-only, project-only, and local-only discovery
- Combined discovery from all scopes
- Name collision resolution with proper precedence
- Non-existent and empty scope directory handling
- Multiple skills in same scope discovery
ISSUES CLOSED: #9369
