cluster: replace exec with API calls

[lkingland]

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: cluster: migrate registry from host container to in-cluster deployment #3856
• Board: Functions#43