Add SKILL.md for affinity diagram methodology

Document the process of creating an affinity diagram for synthesizing qualitative research data.

design-research/skills/affinity-diagram/SKILL.md

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+---
+name: affinity-diagram
+description: Organize qualitative research data into an affinity diagram with themes, clusters, and insight statements. Use when synthesizing large amounts of qualitative data from interviews, observations, or surveys.
+---
+
+# Affinity Diagram
+
+Organize qualitative research data into themed clusters and insight statements.
+
+## Context
+
+You are a UX researcher synthesizing qualitative data for $ARGUMENTS. If the user provides files (interview notes, observation data, survey responses), read them first.
+
+## Instructions
+
+1. **Extract data points**: Pull individual observations, quotes, and notes from the raw data.
+2. 2. **Bottom-up clustering**: Group related data points into natural clusters (do not start with predefined categories).
+   3. 3. **Name each cluster**: Create descriptive theme labels that capture the essence of each group.
+      4. 4. **Create hierarchy**: Organize clusters into higher-level themes (typically 3-5 top-level themes).
+         5. 5. **Write insight statements**: For each theme, write a clear insight statement that captures the "so what?"
+            6. 6. **Identify patterns**: Note frequency, intensity, and connections between themes.
+               7. 7. **Prioritize**: Rank insights by impact on design decisions.
+                 
+                  8. Present the affinity diagram as a structured hierarchy with insight statements and supporting evidence.