feat(http): chi router skeleton + Google-style error response

internal/transport/http/response.go

@@ -0,0 +1,52 @@
+// Package httpx provides the HTTP transport layer: a Chi-based router
+// skeleton and Google API style JSON response helpers (WriteJSON,
+// WriteError). It deliberately uses package name "httpx" to avoid
+// shadowing the standard library "net/http" package at call sites.
+package httpx
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/yan1h/agent-coding-workflow/internal/infra/errs"
+)
+
+// WriteJSON writes body as JSON with the given HTTP status. The
+// Content-Type header is set to application/json before WriteHeader is
+// called. Encode errors are intentionally ignored: by the time the
+// status has been written the connection is committed, and the caller
+// has no meaningful recovery path.
+func WriteJSON(w http.ResponseWriter, status int, body any) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(body)
+}
+
+// errorResponse is the Google API style flat error body.
+type errorResponse struct {
+	Code      string         `json:"code"`
+	Message   string         `json:"message"`
+	Fields    map[string]any `json:"fields,omitempty"`
+	RequestID string         `json:"request_id,omitempty"`
+}
+
+// WriteError translates err into a JSON error response. *errs.AppError
+// values are rendered with their declared Code/Message/Fields and the
+// HTTP status mapped via errs.HTTPStatus. Any other error is reported
+// as a generic 500 internal error so internal details don't leak.
+func WriteError(w http.ResponseWriter, requestID string, err error) {
+	if ae, ok := errs.As(err); ok {
+		WriteJSON(w, errs.HTTPStatus(ae.Code), errorResponse{
+			Code:      string(ae.Code),
+			Message:   ae.Message,
+			Fields:    ae.Fields,
+			RequestID: requestID,
+		})
+		return
+	}
+	WriteJSON(w, http.StatusInternalServerError, errorResponse{
+		Code:      string(errs.CodeInternal),
+		Message:   "internal server error",
+		RequestID: requestID,
+	})
+}