Add kyaml as supported output format for kubeseal

**Which component**: `kubeseal`

**Is your feature request related to a problem? Please describe.**
Kubernetes 1.37 offers stable support for `kyaml` output of resources; see also https://kubernetes.io/blog/2026/08/26/kubernetes-v1-37-release/#kyaml.

As kyaml is a subset of yaml, it can be used as a full replacement for manifests, with the added benefit of being less ambiguous than yaml, hence potentially preventing issues.

As such it would be nice to allow kyaml output from `kubeseal`, to allow one to consistently use this for resources in a Kubernetes project and subsequently enforce this with e.g. CI linters.

**Describe the solution you'd like**
Allow passing `--format kyaml` to `kubeseal` to have the resources being output in the kyaml format.

**Describe alternatives you've considered**
n/a

**Additional context**
https://kubernetes.io/blog/2026/08/11/how-to-pretty-print-kubernetes-yaml-as-kyaml/

Start with the kubeseal CLI and inspect how the --format option selects output formats. Run kubeseal --help and the existing CLI checks for format selection, then verify that --format kyaml emits kyaml resources while existing formats remain unchanged.

Written by the indexing model from the issue text.