cluster: replace exec with API calls

The `pkg/cluster` package still shells out via `exec` for kind, kubectl, and (where still used) docker/podman CLIs. We already use the Kubernetes and Docker APIs extensively elsewhere in func.

Longer-term goal: replace those `exec` calls with library/API usage (kind Go API, client-go apply, docker/podman SDK) the same way builders graduated from wrapping CLIs to library calls.

### Context

Raised by @matejvasek on #3856 (in-cluster registry migration); tracking here so the stepping-stone path stays explicit:

> why we use `exec()` instead of using Go code, kind/kubectl is Go code. Also why call docker via CLI `exec()`? Why not API?

Scripts -> Go with the current structure is the intentional first step; next is replace execs with stdlib/APIs where possible.

### Suggested Stages:

- kind: create/delete/cluster ops via kind's Go API
- kubectl apply / wait: client-go (or existing apply helpers) for resources we already model as objects
- docker/podman host config: daemon.json / registries.conf via API or small pure-Go writers (already partially pure file I/O)

### See also:

- Parent migration PR: https://github.com/knative/func/pull/3856
- Board: Functions#43

Start by inventorying the exec calls in pkg/cluster and read the parent migration PR #3856 alongside the existing API usage in builders. Work through the suggested kind, kubectl, and docker/podman stages; done means the targeted cluster operations use Go APIs or pure-Go file handling instead of CLI exec calls while preserving current behavior.

Written by the indexing model from the issue text.