docs: use default kubeconfig with k3s context name

[xnoto]

What

Updates the access section of KUBECTL.md: local kubectl access now standardizes on the default kubeconfig (~/.kube/config) with a context named k3s, instead of a dedicated per-cluster kubeconfig file.

Why

• Simpler onboarding: one merged context list, no KUBECONFIG juggling or shell exports.
• The context name is purely client-side — the apiserver authenticates the Dex-issued token's email/groups claims (provisioned in cloud-init/k3s/cloud_init.cfg), so renaming has no cluster or state impact.

Impact

• Docs-only change to KUBECTL.md; no OpenTofu, cloud-init, or manifest changes. Break-glass and tunnel-recovery sections are unaffected (they concern the node-local admin kubeconfig, not workstation context naming).

Follow-up

• kustomize-cluster still documents the old pattern in its README ### kubectl access section and docs/kubeconfig.example.yaml (dedicated ~/.kube/makeitworkcloud-k3s.yaml, context makeitworkcloud-k3s). KUBECTL.md links there for the OIDC flow, so a companion PR is needed to align that repo.

[github-actions]

OpenTofu Test

OpenTofu test passed.

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Terraform validate.......................................................Passed
Terraform validate with tflint...........................................Passed
Checkov..................................................................Passed
Terraform fmt............................................................Passed
Terraform docs...........................................................Passed
Detect hardcoded secrets.................................................Passed
check for case conflicts.................................................Passed
check for merge conflicts................................................Passed
check for broken symlinks............................(no files to check)Skipped
check vcs permalinks.....................................................Passed
detect destroyed symlinks................................................Passed
detect private key.......................................................Passed
fix end of files.........................................................Passed
mixed line ending........................................................Passed
trim trailing whitespace.................................................Passed
don't commit to branch..................................................Skipped
check for added large files..............................................Passed

[github-actions]

OpenTofu Plan

OpenTofu plan passed.

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OpenTofu will perform the following actions:

  # module.k3s.libvirt_cloudinit_disk.commoninit will be created
  + resource "libvirt_cloudinit_disk" "commoninit" {
      + id             = (known after apply)
      + meta_data      = <<-EOT
            instance-id: k3s
            local-hostname: k3s
        EOT
      + name           = "k3s_commoninit"
      + network_config = <<-EOT
            version: 2
            ethernets:
              enp1s0:
                dhcp4: true
              enp2s0:
                dhcp4: false
                addresses:
                  - 192.168.102.2/24
        EOT
      + path           = (known after apply)
      + size           = (known after apply)
      + user_data      = (sensitive value)
    }

  # module.runner.libvirt_cloudinit_disk.commoninit will be created
  + resource "libvirt_cloudinit_disk" "commoninit" {
      + id             = (known after apply)
      + meta_data      = <<-EOT
            instance-id: runner
            local-hostname: runner
        EOT
      + name           = "runner_commoninit"
      + network_config = (sensitive value)
      + path           = (known after apply)
      + size           = (known after apply)
      + user_data      = (sensitive value)
    }

Plan: 2 to add, 0 to change, 0 to destroy.
OpenTofu will perform the following actions:

  # module.k3s.libvirt_cloudinit_disk.commoninit will be created
  + resource "libvirt_cloudinit_disk" "commoninit" {
      + id             = (known after apply)
      + meta_data      = <<-EOT
            instance-id: k3s
            local-hostname: k3s
        EOT
      + name           = "k3s_commoninit"
      + network_config = <<-EOT
            version: 2
            ethernets:
              enp1s0:
                dhcp4: true
              enp2s0:
                dhcp4: false
                addresses:
                  - 192.168.102.2/24
        EOT
      + path           = (known after apply)
      + size           = (known after apply)
      + user_data      = (sensitive value)
    }

  # module.runner.libvirt_cloudinit_disk.commoninit will be created
  + resource "libvirt_cloudinit_disk" "commoninit" {
      + id             = (known after apply)
      + meta_data      = <<-EOT
            instance-id: runner
            local-hostname: runner
        EOT
      + name           = "runner_commoninit"
      + network_config = (sensitive value)
      + path           = (known after apply)
      + size           = (known after apply)
      + user_data      = (sensitive value)
    }

Plan: 2 to add, 0 to change, 0 to destroy.