Abort watchpr when a poll can no longer see the PR

- treat a mid-run 404 on a tracked PR as terminal
- cap consecutive transient poll failures at 20 per PR
- reset the failure count on a successful poll
- export IsNotFound for callers to classify the abort

internal/agent/gitea.go

@@ -24,8 +24,10 @@ func (e *APIError) Error() string {
 	return fmt.Sprintf("gitea %s %s: HTTP %d: %s", e.Method, e.Path, e.StatusCode, e.Body)
 }
 
-// isNotFound reports whether err is a Gitea 404.
-func isNotFound(err error) bool {
+// IsNotFound reports whether err is a Gitea 404. Gitea hides repositories a
+// caller may not see behind a 404 rather than a 403, so this also covers a repo
+// that was renamed, deleted, or made private.
+func IsNotFound(err error) bool {
 	var apiErr *APIError
 	return errors.As(err, &apiErr) && apiErr.StatusCode == http.StatusNotFound
 }