refactor: adopt standard Go project layout

[skyoo2003]

Summary

Restructure the repository around the Go project-layout conventions while keeping runtime behavior unchanged.

Type of Change

• Breaking change
• Documentation
• Refactor
• CI/CD

Changes

• Move the public library to pkg/kvs and private RESP codec to internal/resp.
• Keep cmd/kvs as a thin entrypoint and move Cobra/Viper composition to internal/app/kvs.
• Move packaging configuration to build/package and the Hugo site to website.
• Update imports, automation, ownership rules, documentation, and Git ignore rules.

Changelog

• changie new fragment added — one sentence, two at most, 280 characters max
• Not user-visible, no fragment needed
• No fragment was added because no related issue number was supplied; the import-path migration needs a release-note decision.

Testing

• make all passes
• Existing tests moved with their packages; no new behavior requires tests
• Hugo site build and GoReleaser configuration validation pass

Related Issues

None.