test(config-audit): add marketplace-small/medium/large scanner fixtures

plugins/config-audit/tests/fixtures/marketplace-medium/.claude/shared.md

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+# Shared Medium Patterns
+
+Naming conventions and common helpers shared across the four plugins.
+
+## Naming
+
+- `camelCase` for variables and functions
+- `PascalCase` for classes and types
+
+## Error Handling
+
+- Early returns over nested conditionals
+- Typed error subclasses