fix(lint): resolve all 29 errcheck violations

[shantoislamdev]

Fix

Resolves all 29 errcheck violations flagged by golangci-lint:

• defer Close(): Replaced bare defer file.Close() with defer func() { _ = file.Close() }() across commands.go, init.go, generator.go, sampler.go, jsonl_test.go
• fmt.Fprintln/Fprintf/w.Flush: Added explicit _ = for tabwriter output in commands.go (schema list, schema show, provider list)
• provider.CloseAll/bar.Finish: Wrapped in _ = or deferred closure
• os.Setenv/Unsetenv in tests: Replaced with .Setenv() (auto-cleanup, Go 1.17+) in secrets_test.go
• test helpers: Added _ = to gen.Run, r.Register, os.Chmod, w.Close in test files
• unsafe type assertion: Added comma-ok form in chat_test.go
• tmpFile.Close on error paths: Added _ = for discarded close calls in generator.go