Add automation-over-documentation guidance to writing-skills

Mechanical constraints should be automated, not documented—save skills
for judgment calls.

Based on insight from @EthanJStark in PR #146.

skills/writing-skills/SKILL.md

@@ -56,6 +56,7 @@ The entire skill creation process follows RED-GREEN-REFACTOR.
 - One-off solutions
 - Standard practices well-documented elsewhere
 - Project-specific conventions (put in CLAUDE.md)
+- Mechanical constraints (if it's enforceable with regex/validation, automate it—save documentation for judgment calls)
 
 ## Skill Types