feat(tools): add find_references tool

[CalvinAllen]

Summary

Add a new MCP tool find_references that finds all usages of a symbol at a given position.

Details

• Input: file path, line, column, maxResults
• Uses DTE.Find object for solution-wide search
• Filters results to reduce false positives
• Returns ReferencesResult with list of locations

Implementation Tasks

• Add FindReferencesAsync(string path, int line, int column, int maxResults) to IVisualStudioRpc in RpcContracts.cs
• Add FindReferencesAsync to IVisualStudioService.cs
• Implement FindReferencesAsync in VisualStudioService.cs
• Get symbol name at specified position
• Configure DTE.Find for solution-wide search
• Execute search and collect results
• Add filtering logic to reduce false positives
• Implement maxResults limiting with Truncated flag
• Add forwarding method in RpcServer.cs
• Add proxy method in RpcClient.cs
• Add find_references tool method to NavigationTools.cs
• Build and verify no errors

Limitations

• Text-based search may have false positives in comments/strings
• Performance may be slow on large solutions (mitigated with maxResults)

Dependencies

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

Notes

This is Phase 4 of the LSP-style navigation tools feature. Most complex tool, benefits from patterns established in earlier phases.