refactor: move Go module from localhost/leetcode to repo root

[eselyavka]

Summary

• Move module to repo root and set path to github.com/eseliavka/go
• Move problems/, util/, .golangci.yml, go.mod, go.sum to root
• Update imports and all scripts/workflows/docs for root layout
• Remove go.work, go.work.sum, old nested module files, and obsolete localhost/ directory
• Verification: make verify passes from repo root