feat: patterns/ directory + skill-impact tracker (WikiSkill-inspired)

- New patterns/ directory with 7 initial failure-mode patterns (PAT-001..007)
- skill-impact.md audit trail for skill modifications
- _template.md for future pattern creation
- index.md updated with Patterns section
- log.md entry for this change
- Inspired by arXiv:2608.27454 (WikiSkill)

patterns/_template.md

@@ -0,0 +1,30 @@
+---
+pattern_id: PAT-XXX
+title: "<short descriptive title>"
+category: database|infrastructure|storage|networking|tooling|integration|security
+severity: low|medium|high
+status: active|superseded
+first_observed: YYYY-MM
+last_updated: YYYY-MM-DD
+related_systems: []
+related_solution_docs: []
+related_skills: []
+---
+
+# <Title>
+
+## Symptom
+What goes wrong? What are the observable symptoms?
+
+## Root Cause
+Why does it happen? Trace the actual cause, not just the symptom.
+
+## Mitigation
+What was the fix? Include commands/snippets if relevant.
+
+## Prevention
+How to avoid this in the future? (monitoring, lint rule, convention, etc.)
+
+## Evidence
+- When was this observed? Which incidents?
+- Links to solution docs, session IDs, MEMORY entries