fix: use resource-root path for update_project_uids

[vivienvoid]

Summary

• stop passing the absolute host project path into resave_resources
• pass res:// so the Godot-side operation stays rooted in the active project
• preserve the existing project validation and execution flow

Why

update_project_uids previously passed an absolute host path like /mnt/c/... into the Godot-side resave_resources() operation. That script prefixes non-resource paths with res://, which turned the path into malformed res:///mnt/... and caused the tool to process zero files while still reporting success.

This change keeps the operation rooted in the active project by passing res:// explicitly instead.

Verification

• npm run build
• live MCP probe against Godot 4.6.2.stable.official.71f334935
• confirmed update_project_uids now reports:
  • Using project path: res://
  • Found 3 scenes
  • Found 1 scripts/shaders

Closes #102