✨ feat: implement input validation for API v2

- Added go-playground/validator dependency

- Created pkg/validation/ package with custom validator wrapper

- Implemented request validation for v2 greet endpoint

- Added structured validation error responses

- Extended BDD tests to cover validation scenarios

- Updated AGENTS.md with v2 API documentation

- Created ADR 0011-validation-library-selection.md

- Simplified server handler creation code

- Updated CHANGELOG with implementation details

CHANGELOG.md

@@ -62,6 +62,17 @@ vibe start --agent dancelessonscoachprogrammer
 - ✅ Updated configuration system to support API versioning
 - ✅ Added comprehensive test coverage for both enabled and disabled states
 
+### 2026-04-04 - Input Validation Implementation
+- ✅ Selected go-playground/validator for input validation
+- ✅ Created ADR [0011-validation-library-selection.md](adr/0011-validation-library-selection.md)
+- ✅ Added `pkg/validation/` package with custom validator wrapper
+- ✅ Implemented request validation for v2 API endpoints
+- ✅ Added structured validation error responses
+- ✅ Extended BDD tests to cover validation scenarios
+- ✅ Added validation for name field (max length: 100 characters)
+- ✅ Maintained graceful degradation when validator fails to initialize
+- ⚠️ **REMINDER**: Use `./scripts/build.sh` instead of `go build` directly for consistent builds
+
 ## Compact History (Last 5 Entries)
 
 ### 2026-04-04