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

packages/core/src/application/AGENTS.md

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+# Application Layer — Use Cases + Interfaces
+
+## Rules
+
+- Imports from entities/ ONLY
+- NEVER imports from infrastructure/ or interface-adapters/
+- Repository interfaces define data access contracts
+- Service interfaces define external service contracts
+- Use cases get dependencies via `getInjection()` — never direct import of implementations
+
+## Adding a New Use Case
+
+1. Create `src/application/use-cases/{domain}/{verb}-{noun}.use-case.ts`
+2. Get dependencies via DI: `const repo = getInjection("IMyRepository")`
+3. Implement business logic using entities and interfaces only
+4. Write test in `tests/unit/use-cases/{domain}/` using `initializeContainer()`/`destroyContainer()` pattern
+
+## Adding a New Repository Interface
+
+1. Create `src/application/repositories/{name}.repository.interface.ts`
+2. Define interface methods returning entity types
+3. Export from `src/application/repositories/index.ts`
+4. Create mock implementation in infrastructure/
+5. Register in DI container (add symbol, module binding)
+
+## Adding a New Service Interface
+
+Same as repository, but in `src/application/services/`

packages/core/src/application/use-cases/auth/AGENTS.md

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+# Auth Domain — Business Rules
+
+## Responsibility
+
+Authentication and authorization: sign-in, sign-up, sign-out, session management.
+
+## Business Rules
+
+- Passwords are hashed via IAuthenticationService (never stored plain)
+- Sessions expire after 7 days (configured in mock, real impl may differ)
+- Sign-up requires unique username
+- Sign-in verifies password via IAuthenticationService.verifyPassword()
+- Sign-out invalidates session and returns blank cookie
+
+## Error Cases
+
+- `AuthenticationError` — wrong credentials (sign-in) or username taken (sign-up)
+- `UnauthenticatedError` — invalid/expired session
+
+## Dependencies
+
+- `IUsersRepository` — user lookup and creation
+- `IAuthenticationService` — password hashing, session management
+
+## Adding a New Auth Use Case
+
+1. Create `{verb}-{noun}.use-case.ts` in this folder
+2. Get deps via `getInjection("IUsersRepository")`, `getInjection("IAuthenticationService")`
+3. Write test first in `tests/unit/use-cases/auth/`
+4. Use `initializeContainer()`/`destroyContainer()` pattern in tests

packages/core/src/application/use-cases/content/AGENTS.md

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+# Content Domain — Business Rules
+
+## Responsibility
+
+Article management: creation, retrieval, publishing workflow.
+
+## Business Rules
+
+- Articles must have a title and content
+- Slugs auto-generated from title if not provided
+- New articles default to "draft" status
+- Filtering by status, authorId supported
+- Pagination via limit/offset
+
+## Error Cases
+
+- `NotFoundError` — article doesn't exist (future: update/delete)
+- `UnauthorizedError` — user can't edit article (future)
+- `InputParseError` — missing required fields (handled by controller)
+
+## Dependencies
+
+- `IArticlesRepository` — article CRUD operations
+
+## Adding a New Content Use Case
+
+1. Create `{verb}-{noun}.use-case.ts` in this folder
+2. Get deps via `getInjection("IArticlesRepository")`
+3. Write test first in `tests/unit/use-cases/content/`