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

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

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+---
+paths: src/**/*.ts
+---
+
+# TypeScript Rules
+
+Use strict TypeScript throughout.
+
+## Mandatory
+
+- `strict: true` in tsconfig
+- No `any` — prefer `unknown` at boundaries
+- Prefer `type` aliases for simple shapes, `interface` for extendable objects

plugins/config-audit/tests/fixtures/baseline-all-a/.claude/settings.json

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+{
+  "$schema": "https://json.schemastore.org/claude-code-settings.json",
+  "permissions": {
+    "allow": ["Bash(npm run *)", "Read(src/**)"],
+    "deny": ["Read(./.env)"]
+  },
+  "effortLevel": "medium"
+}

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