fix: enforce checked cleanup and output handling

[zzwong]

Summary

• handle previously unchecked output, response-body, archive, and temporary-file cleanup errors
• add a minimal golangci-lint v2 configuration with no broad suppressions
• run pinned golangci-lint v2.12.2 for code changes and include it in the required CI gate
• add a dynamic maintained lint badge and update contributor commands
• add end-to-end synthetic tests for Green Button commands, Opower usage resources, realtime reads, summaries, and entity discovery
• raise total statement coverage from 61.6% to 71.0% and enforce a 70% floor

Testing

• make check
• govulncheck ./...
• golangci-lint config verify
• actionlint .github/workflows/*.yml
• total statement coverage: 71.0%

Security and privacy

• no production fixtures or provider values changed
• all new provider responses are synthetic
• cleanup errors are either propagated or explicitly ignored only on best-effort cleanup paths