docs: add CLAUDE.md with architecture guidelines and code size limits

CLAUDE.md

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+# Architecture Guidelines
+
+## MVVM Structure (Kotlin Android)
+
+```
+android/app/src/main/java/com/cardsnap/
+├── data/            # Room DB, DAO, Repositories
+├── domain/          # Models, OCR, Parser
+├── ui/              # Screens, Navigation, Theme
+├── util/            # Utilities
+└── MainActivity.kt
+```
+
+## Dependency Rules
+
+- `ui` → `domain`, `data` (via ViewModel)
+- `domain` → NO external dependencies
+- `data` → `domain` (models)
+- ViewModels mediate between UI and data layers
+
+## Naming Conventions
+
+- Screens: `*Screen.kt`
+- ViewModels: `*ViewModel.kt`
+- Repositories: `*Repository.kt`
+- DAOs: `*Dao.kt`
+- Models: `*Model.kt` or data classes
+- Utils: `*Util.kt` or `*Helper.kt`
+
+## Code Size Limits
+
+- **Max 200 lines per file** (warn at 200, error at 300)
+- **Max 30 lines per function** (warn at 30, error at 50)
+- **Complexity ≤ 8** (warn), ≤ 10 (error)
+- **Max nesting depth ≤ 4**
+- **Max 3 parameters per function**
+
+If code exceeds these limits, REFACTOR immediately using Extract Method, Extract Class, or other patterns.
+
+## Forbidden Patterns
+
+- NO direct Room/DB access from UI layer
+- NO business logic in Composables
+- NO circular dependencies between modules
+- All external deps go through repositories
+
+## Before Generating Code
+
+1. Identify which layer the code belongs to
+2. Check dependency rules
+3. Keep composables focused on UI only
+4. Move business logic to ViewModels
+5. Keep files under 200 lines, functions under 30 lines
+6. Run: `./gradlew lint` before commit