feat: add release check helpers

[vincentkoc]

Summary

• add a shared releasecheck package for GitHub latest-release polling, 24h cache reads, semver-ish comparison, and update notice formatting
• skip passive notices for JSON output, CI, non-terminal stderr, metadata/check-update commands, disabled env vars, and dev/dirty builds
• document the shared ownership boundary and downstream opt-in pattern

Verification

• GOWORK=off go mod tidy
• git diff --exit-code -- go.mod go.sum
• GOWORK=off go vet ./...
• GOWORK=off go test -count=1 ./...
• git diff --check