Progressive Disclosure Skills Plugin (SuperPi.Extension behaviour).

Implements progressive disclosure skill loading semantics:

• Discovers skills across workspace, global (<global config>/skills), and custom skill locations.
• Uses progressive disclosure: injects only - <name> (<SKILL.md path>): <description> into a dedicated pi.system section (id: "skills_index") so the model's context window is not flooded with full skill bodies until the model reads SKILL.md on demand.