feat(core): add 8 AGENTS.md files (package, layer, domain levels)

packages/core/src/entities/AGENTS.md

@@ -0,0 +1,33 @@
+# Entities Layer — Innermost, Zero Dependencies
+
+## Rules
+
+- NEVER import from application/, infrastructure/, interface-adapters/, or di/
+- NEVER import external libraries except Zod (for schema validation)
+- Everything here is pure — no side effects, no I/O, no async
+- Models are Zod schemas with inferred TypeScript types
+- Errors are custom Error subclasses with domain-specific semantics
+
+## Adding a New Model
+
+1. Create `src/entities/models/{name}.ts`
+2. Define Zod schema and export inferred type:
+   ```typescript
+   import { z } from "zod";
+   export const {name}Schema = z.object({ ... });
+   export type {Name} = z.infer<typeof {name}Schema>;
+   ```
+3. Export from `src/entities/models/index.ts`
+
+## Adding a New Error
+
+1. Create or edit `src/entities/errors/{domain}.ts`
+2. Extend Error with constructor accepting message + options:
+   ```typescript
+   export class {Name}Error extends Error {
+     constructor(message: string, options?: ErrorOptions) {
+       super(message, options);
+     }
+   }
+   ```
+3. Export from `src/entities/errors/index.ts`