fix(golangci): treat empty stdout as no-issues and surface stderr

[notJoon]

Summary

• Stop emitting decode golangci-lint output: unexpected end of JSON input warnings for every .gno file in a package.
• golangci-lint exits with empty stdout when go/packages cannot load the target (the common case is a pure-.gno directory whose temp .go file imports gno.land/... paths the standard Go loader cannot resolve). The previous code passed that empty body to json.Unmarshal, which failed and the engine logged a warn per file.
• Short-circuit empty (or whitespace-only) stdout to a zero result with no error. Capture stderr separately and embed a truncated snippet in the wrapped error so real decode failures stay diagnosable.
• Extract parsing into decodeGolangciOutput so empty / valid / malformed branches are unit-testable without spawning the binary.

Repro

CI on a pure-.gno repo (e.g. gnoswap-labs/gnoswap run 24982317573) shows hundreds of decode golangci-lint output for <dir>: unexpected end of JSON input warnings — one per .gno file. After this change those warnings disappear; real decode failures still surface with the stderr snippet attached.

Test plan

• go build ./...
• make test (race + shuffle) — all packages pass
• Bench: go test -bench=BenchmarkRun -benchmem -run=^$ -benchtime=3x ./internal/ within ±5% of main
• New default_golangci_test.go: empty / valid / malformed JSON for decodeGolangciOutput, plus 6 cases for snippet
• Re-run tlin against gnoswap and confirm the per-file warnings are gone

Follow-up

A separate PR will wire GOPACKAGESDRIVER=gnopls so golangci-lint can actually analyze .gno packages instead of producing empty output.