test(config-audit): add baseline-all-a fixture + grade-stability regression test

plugins/config-audit/tests/fixtures/baseline-all-a/.claude/shared.md

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+# Shared Configuration
+
+Common patterns and conventions shared across the baseline fixture.
+
+## Naming Conventions
+
+- `camelCase` for variables and functions
+- `PascalCase` for classes, interfaces, and types
+- `UPPER_SNAKE_CASE` for module-level constants
+
+## Error Handling
+
+- Prefer early return over deep nesting
+- Throw `Error` subclasses with typed messages
+- Never swallow errors silently