Remove deprecated agent files and add new architecture, contract, and feature documentation for the Reader Suite

ARCHITECTURE.md

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+# Reader Suite Architecture (Web)
+
+Tai lieu nay mo ta vai tro cua `reader` (web app) trong bo 3 he thong: Web + Android + API.
+
+## Vai tro trong he thong
+
+- `reader` la frontend web cho end-user va mot phan workflow cho MOD/ADMIN.
+- `reader` KHONG duoc tro thanh noi xu ly business logic chinh.
+- `reader-api` la API trung tam, la nguon su that cho contract va domain behavior.
+
+## Nguyen tac thong nhat voi mobile + backend
+
+- API-first: moi tinh nang user-facing moi phai co endpoint ro rang trong `reader-api`.
+- Contract-first: web va mobile dung chung schema response, error code, pagination.
+- Auth parity:
+  - Web: NextAuth session cookies.
+  - Mobile: Bearer JWT.
+  - Ca hai deu map vao cung user identity trong backend.
+- Data ownership:
+  - PostgreSQL: metadata co cau truc (user, novel, genre, comment, bookmark...).
+  - MongoDB: chapter content, recommendation payload lon.
+
+## Kien truc module web
+
+- App layer (`app/*`): route, rendering, page composition.
+- UI layer (`components/*`): reusable components, khong chua business rule quan trong.
+- Data access layer: goi REST API qua `READER_API_ORIGIN` cho endpoint da migrate.
+- Auth adapter: dong bo session NextAuth va profile API.
+
+## Quy uoc tich hop API
+
+- Base URL local: `http://localhost:8000` (qua `READER_API_ORIGIN`).
+- Khong hard-code endpoint trong component; gom theo domain (novels, chapters, user...).
+- Luon xu ly 3 nhom loi:
+  - validation/business (`4xx`)
+  - auth (`401/403`)
+  - he thong (`5xx`)
+
+## Definition of Done (Web)
+
+- Tinh nang moi co endpoint tai `reader-api` hoac da duoc architect chap thuan.
+- Khong duplicate business logic voi mobile.
+- Da test luong auth va state dong bo profile/bookmark/progress.
+- README va contract docs duoc cap nhat neu co thay doi API.