Add persuasion research foundation and enforce TDD for skill edits

Based on Meincke et al. (2025) research showing LLMs respond to same persuasion
principles as humans. Created reference doc explaining psychology behind effective
skill design (authority, commitment, scarcity, social proof, unity).

Extended Iron Law to explicitly cover skill edits - agents were skipping
RED-GREEN-REFACTOR when editing existing skills ("just a simple addition").

Changes:
- Add persuasion-principles.md reference doc with research citations
- Update creating-skills when_to_use to include editing skills
- Expand Iron Law with "Edit skill without testing? Same violation"
- Add explicit no-exceptions for "simple additions", "documentation updates"
- Add psychology note reference in bulletproofing section
- Add persuasion psychology section to testing-skills-with-subagents

Baseline testing showed agents already apply persuasion principles intuitively
when creating skills. This change makes the WHY explicit and prevents
rationalization that edits don't need testing.

skills/meta/testing-skills-with-subagents/SKILL.md

@@ -144,6 +144,8 @@ Forces explicit choice.
 
 **Best tests combine 3+ pressures.**
 
+**Why this works:** See skills/meta/creating-skills/persuasion-principles.md for research on how authority, scarcity, and commitment principles increase compliance pressure.
+
 ### Key Elements of Good Scenarios
 
 1. **Concrete options** - Force A/B/C choice, not open-ended