Add centralized path validation to prevent path traversal attacks.
- PathValidator class with sync/async validation methods
- Protects against '..' and '~' traversal patterns
- Validates paths are within project root
- Refactored all 7 file tools to use PathValidator
- 51 new tests for PathValidator
- ToolRegistry: tool lifecycle management, execution with validation
- GetLinesTool: read file lines with line numbers
- GetFunctionTool: get function source using AST
- GetClassTool: get class source using AST
- GetStructureTool: directory tree with filtering
121 new tests, 540 total