docs: add Agent Rules section to IMPLEMENTATION.md

Added key principles and constraints:
- React 19 optimization (no useCallback/useMemo)
- Code conventions (no comments, type-first)
- Architecture patterns (hook composition, refs)
- Documentation standards

These rules guide all implementation decisions and ensure consistency.

IMPLEMENTATION.md

@@ -8,6 +8,35 @@ We are porting vim-sudoku from the old architecture (src_old) to follow the unif
 
 ---
 
+## Agent Rules
+
+These are the key principles and constraints that guide this implementation:
+
+### React 19 Optimization
+- **No `useCallback` or `useMemo`**: React 19's compiler automatically optimizes, making these hooks unnecessary
+- Use plain functions instead of wrapped callbacks
+- React will handle memoization automatically
+- Cleaner, more readable code
+
+### Code Conventions
+- **No comments**: Code should be self-documenting unless explicitly requested
+- **Type-first approach**: Define types before implementation
+- **Explicit imports**: Always import types with `import type`
+- **Return type annotations**: Use `ReturnType<typeof hookName>` for hook return types
+
+### Architecture Patterns
+- **Hook composition**: Small, focused hooks composed together
+- **Ref-based state**: Use refs for non-reactive state, useState for reactive
+- **Module organization**: Each hook module has types.ts, implementation, and index.ts
+- **Follow vim-maze patterns**: Maintain consistency across games
+
+### Documentation
+- **Update IMPLEMENTATION.md**: After every iteration
+- **Clear commit messages**: Describe what, why, and how
+- **Track issues and solutions**: Document problems encountered and fixes
+
+---
+
 ## Iteration 1: Preparation & Planning
 
 **Date**: 2025-01-22