fix: resolve linter errors

[marcus]

Summary

Fixed all linting errors reported by golangci-lint across 3 files:

• 8 errcheck violations (unchecked fmt.Fprintf/Fprintln and os.Setenv calls, w.Close in tests)
• 1 staticcheck violation (unnecessary embedded field selector in Duration.MarshalJSON)

Changes

run.go (9 fixes):

• displayPreflight: Assign all fmt.Fprintf and fmt.Fprintln error returns to _ (these are safe expected operations writing to stdout)
• ensurePATH: Assign os.Setenv error to _

run_test.go (4 fixes):

• TestConfirmRun_TTYAcceptsY, TTYAcceptsYes, TTYDefaultRejectsEmpty: Assign w.Close() error to _ in pipe cleanup
• captureStdout helper: Assign w.Close() error to _

stats.go (1 fix):

• MarshalJSON: Remove redundant embedded field selector (d.Duration.Seconds → d.Seconds)

Testing

• All unit tests pass
• golangci-lint reports no errcheck or staticcheck violations

🤖 Generated with Claude Code