feat(veans): add stable error envelope and code constants

veans/internal/output/errors.go

@@ -0,0 +1,69 @@
+// Package output defines stable error codes and the JSON envelope
+// veans uses for non-zero exits.
+package output
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+)
+
+type Code string
+
+const (
+	CodeNotFound            Code = "NOT_FOUND"
+	CodeConflict            Code = "CONFLICT"
+	CodeValidation          Code = "VALIDATION_ERROR"
+	CodeAuth                Code = "AUTH_ERROR"
+	CodeRateLimited         Code = "RATE_LIMITED"
+	CodeBotUsersUnavailable Code = "BOT_USERS_UNAVAILABLE"
+	CodeNotConfigured       Code = "NOT_CONFIGURED"
+	CodeUnknown             Code = "UNKNOWN"
+)
+
+// Error is the structured error type used for both internal flow and the
+// `--json` mutation envelope. Callers wrap underlying errors with codes via
+// Wrap; the cobra runner converts unmapped errors to CodeUnknown.
+type Error struct {
+	Code    Code   `json:"code"`
+	Message string `json:"error"`
+	Cause   error  `json:"-"`
+}
+
+func (e *Error) Error() string { return e.Message }
+func (e *Error) Unwrap() error { return e.Cause }
+
+func New(code Code, format string, args ...any) *Error {
+	return &Error{Code: code, Message: fmt.Sprintf(format, args...)}
+}
+
+func Wrap(code Code, cause error, format string, args ...any) *Error {
+	return &Error{Code: code, Message: fmt.Sprintf(format, args...), Cause: cause}
+}
+
+// AsError extracts an *Error from any error chain, returning a CodeUnknown
+// wrapper for plain errors.
+func AsError(err error) *Error {
+	if err == nil {
+		return nil
+	}
+	if e, ok := err.(*Error); ok {
+		return e
+	}
+	return &Error{Code: CodeUnknown, Message: err.Error(), Cause: err}
+}
+
+// EmitError writes the JSON envelope when --json is set, or a plain message
+// otherwise. Always to stderr so stdout stays parseable.
+func EmitError(jsonMode bool, err error, w io.Writer) {
+	if w == nil {
+		w = os.Stderr
+	}
+	e := AsError(err)
+	if jsonMode {
+		_ = json.NewEncoder(w).Encode(e)
+		return
+	}
+	fmt.Fprintf(w, "veans: %s: %s\n", e.Code, e.Message)
+}