feat(errkind): Of -- a kind from a fixed vocabulary, never the error's text (review F3, F5)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/errkind/errkind.go

@@ -0,0 +1,42 @@
+// Package errkind reduces an error to a short kind that is safe to log, store and show.
+//
+// An error's own text can hold a provider's response body (a gateway that echoes the
+// prompt), a file path (ffmpeg names the spool file), or a word from a private
+// transcript. None of that may reach a log line, a database column or a page about a
+// private recording (ADR-033: "typed, never a provider's string"; ADR-035). Of returns
+// only a fixed vocabulary, so it can never carry any.
+package errkind
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+)
+
+// HTTPStatuser is implemented by errors that are a non-2xx answer from a service.
+type HTTPStatuser interface{ HTTPStatus() int }
+
+// Of is the kind of err: "timeout", "canceled", "http_<code>", "network", or "error".
+// It is empty for nil.
+func Of(err error) string {
+	if err == nil {
+		return ""
+	}
+	var hs HTTPStatuser
+	var ne net.Error
+	switch {
+	case errors.Is(err, context.DeadlineExceeded):
+		return "timeout"
+	case errors.Is(err, context.Canceled):
+		return "canceled"
+	case errors.As(err, &hs):
+		return fmt.Sprintf("http_%d", hs.HTTPStatus())
+	case errors.As(err, &ne):
+		if ne.Timeout() {
+			return "timeout"
+		}
+		return "network"
+	}
+	return "error"
+}