feat(cli): publish artifacts to imgsrv

[jmgilman]

Summary

• implement imgcli publish CONFIG as build plus imgsrv upload
• add publish-only imgsrv config flags/env keys and digest-only stdout
• add multipart uploader with best-effort abort before completion succeeds
• keep committed go.work for ././schemas and use ignored go.work.local for sibling imgsrv development

Testing

• go test ./...
• moon ci --summary minimal
• GOWORK=off go test ./...
• GOWORK=off moon ci --summary minimal
• go mod tidy -diff
• git diff --cached --check