Add multi-file refactoring support

[patmakesapps]

Problem

Renaming a symbol across an entire codebase is tedious and error-prone manually. LumaKit already has a symbol table that could power intelligent multi-file refactoring.

Proposed Solution

• Use find_usages to locate all references
• Safe rename with preview option
• Handle imports and exports correctly
• Could extend to "extract function" refactoring

Acceptance Criteria

• Can rename a symbol across all files in the repo
• Shows preview of all changes before applying
• Handles imports and exports correctly
• No broken references after rename