feat(tools): add goto_definition navigation tool

[CalvinAllen]

Summary

Add a new MCP tool goto_definition that finds where a symbol at a given position is defined.

Details

• Input: file path, line, column
• Uses DTE.ExecuteCommand("Edit.GoToDefinition") after positioning cursor
• Captures resulting document/position as definition location
• Returns DefinitionResult with location(s) and symbol info

Implementation Tasks

• Add GoToDefinitionAsync(string path, int line, int column) to IVisualStudioRpc in RpcContracts.cs
• Add GoToDefinitionAsync to IVisualStudioService.cs
• Implement GoToDefinitionAsync in VisualStudioService.cs
• Handle document opening and cursor positioning
• Execute Edit.GoToDefinition command
• Capture resulting location
• Handle edge cases (no definition found, external code)
• Add forwarding method in RpcServer.cs
• Add proxy method in RpcClient.cs
• Add goto_definition tool method to NavigationTools.cs
• Build and verify no errors

Limitations

• Cannot navigate to external/NuGet assemblies without source
• Partial classes may only return one location

Dependencies

• Requires feat(tools): add symbol_document navigation tool #13 (symbol_document) for SymbolModels DTOs

Notes

This is Phase 3 of the LSP-style navigation tools feature.