feat(filesystem): add compare_files tool for file diff comparison

[factspark23-hash]

What

Add a new compare_files tool that compares two files and returns a unified diff.

Why

Currently there is no way to compare two files through MCP. Users often need to:

• Verify an edit was applied correctly
• Understand differences between file versions
• Review changes before committing

Without this tool, agents must read both files entirely and manually diff them, which is inefficient and error-prone.

How

compare_files({
  file1: "path/to/original.ts",
  file2: "path/to/modified.ts",
  contextLines: 3  // optional, default 3
})

Returns unified diff output, or reports files as identical.

Changes

• src/filesystem/index.ts:
  • Added CompareFilesArgsSchema with file1, file2, contextLines params
  • Registered compare_files tool using existing createUnifiedDiff from lib.ts
  • Both files must be within allowed directories