Add Claude Code plugin configuration for analysis tools

- Add plugin auto-install configuration in .claude/settings.json
- Configure auto-install of analysis-tools plugin from PolicyEngine/policyengine-claude
- Provides 3 skills for PolicyEngine calculator/analysis workflows

Included skills:
- policyengine-us-skill: Simulation patterns, situation creation, variable reference
- policyengine-analysis-skill: Impact analysis, Streamlit dashboards, Plotly visualizations
- policyengine-standards-skill: Code quality, formatting, CI requirements

Benefits:
- Automated reference documentation for PolicyEngine patterns
- Best practices for calculator apps and analysis
- Consistent code quality and visualization standards

Author: MaxGhenis

.claude/settings.json

@@ -0,0 +1,10 @@
+{
+  "plugins": {
+    "marketplaces": [
+      "PolicyEngine/policyengine-claude"
+    ],
+    "auto_install": [
+      "analysis-tools@policyengine-claude"
+    ]
+  }
+}