docs: add hierarchical AGENTS.md for subdirectories

AGENTS.md

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+# AGENTS.md - Fengling.Platform
+
+**Parent:** `/AGENTS.md`
+
+## Overview
+
+Cross-service platform infrastructure — multi-tenant, Identity, authentication.
+
+## Structure
+
+```
+src/Fengling.Platform/
+├── Fengling.Platform.Domain/
+│   └── AggregatesModel/
+│       ├── UserAggregate/     # ApplicationUser
+│       ├── RoleAggregate/     # ApplicationRole
+│       └── TenantAggregate/   # Tenant, TenantInfo
+└── Fengling.Platform.Infrastructure/
+    ├── PlatformDbContext.cs   # IdentityDbContext
+    ├── ITenantStore.cs
+    ├── ITenantManager.cs
+    └── Configurations/        # EF Core configs
+```
+
+## Where to Look
+
+| Task | Location |
+|------|----------|
+| Tenant mgmt | `Platform.Domain/AggregatesModel/TenantAggregate/` |
+| User/Role | `Platform.Domain/AggregatesModel/*Aggregate/` |
+| DB Context | `Platform.Infrastructure/PlatformDbContext.cs` |
+| Auth config | `Platform.Infrastructure/` |
+
+## Conventions
+
+- **Pattern**: Manager + Store (ASP.NET Core Identity style)
+- **Entities**: 贫血模型 for Tenant, 充血模型 for aggregates
+- **ID type**: `long` for all identities
+
+## Anti-Patterns
+
+- **Migration in progress** — Console not yet migrated to Platform
+- Don't add new entities to Console — use Platform instead