fix(skills): add Anti-Patterns sections, fix descriptions, quality checks, and required inputs

- Add Anti-Patterns section (3-5 binary checkboxes) to all modified skills
- Fix Quality Checks to use binary checkbox format where needed
- Rewrite descriptions to verb-when-produces format where needed
- Add Required Inputs sections to skills missing them
- Fix email-triage frontmatter YAML quoting

https://claude.ai/code/session_01MuGKn3a3Gbqoe8uM5Lmuqt

skills/cohort-analysis/SKILL.md

@@ -178,6 +178,14 @@ ORDER BY 1, 3;
 - [ ] Recommendations are tied to specific cohort or segment findings — not generic growth advice
 - [ ] Leading indicators are observable in production data, not just in theory
 
+## Anti-Patterns
+
+- [ ] Do not allow the same user to appear in multiple cohorts — overlapping cohorts produce retention numbers that cannot be compared or acted upon
+- [ ] Do not assume assumed ARPU in LTV projections — use observed revenue per retained user per period, not a blended average that hides segment differences
+- [ ] Do not draw conclusions from cohorts too small to be statistically meaningful — flag minimum cohort size thresholds and note when a cohort is too small to trust
+- [ ] Do not conflate retention rate with engagement rate — a user who logs in but does not complete the key retention event is not retained by the definition used
+- [ ] Do not make recommendations without connecting them to specific cohort or segment findings — generic growth advice that could apply to any product adds no value
+
 ## Example Trigger Phrases
 
 - "Run a cohort analysis for our SaaS product"