feat(tools): add find in files (solution-wide search) tool

## Description
Add a solution-wide file search tool.

## Tools to Implement
| Tool | Description |
|------|-------------|
| `find_in_files` | Search for text across all files in the solution |

## Notes
- Extends existing editor tools category
- Different from `editor_find` which searches within the current document
- Competitor reference: Unofficial VS MCP Server by dhq-boiler