Define the skill entity for generation and curation

[Neverdecel]

Summary

• Define a skill as one on-demand job whose description is the discovery index and whose body is the confirmed procedure the agent would get wrong without it.
• Project that entity into always-on guidance, mining, and curation; stay silent on AGENTS.md, agents, and commands.
• Tighten generated descriptions (what+when, never Use ONLY when) and tests/scenarios against that shape.

Test plan

• npm run check
• npm test
• Live eval remains opt-in (EVAL_CASE=4 for save-quality).