feat: provision register command and k3s kubeconfig merge

Add `labctl provision register` to re-add machines to installed state
without reprovisioning (e.g. after bastion state loss). Full stack:
protocol type, bastion API + WS handler, labd route, CLI command.

Add `labctl app k3s kubeconfig <target>` to fetch kubeconfig from a
k3s node via SSH, rewrite server URL, and merge into ~/.kube/config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bastion/completions/labctl.fish

@@ -124,6 +124,7 @@ complete -c labctl -n "__labctl_using_cmd provision" -a install -d 'Queue a disc
 complete -c labctl -n "__labctl_using_cmd provision" -a reprovision -d 'Queue install + SSH reboot into PXE (target: hostname, MAC, or IP)'
 complete -c labctl -n "__labctl_using_cmd provision" -a debug -d 'PXE boot into Fedora rescue mode for debugging (target: hostname, MAC, or IP)'
 complete -c labctl -n "__labctl_using_cmd provision" -a forget -d 'Remove a machine from bastion state'
+complete -c labctl -n "__labctl_using_cmd provision" -a register -d 'Register an already-installed machine (e.g. after state loss)'
 complete -c labctl -n "__labctl_using_cmd provision" -a logs -d 'Show provisioning logs for a machine (hostname, MAC, or IP)'
 complete -c labctl -n "__labctl_using_cmd provision" -a makeiso -d 'Generate a UEFI-bootable iPXE ISO for network provisioning'
 
@@ -140,6 +141,10 @@ complete -c labctl -n "__labctl_in_cmd provision reprovision" -l disk -d 'Target
 # provision debug options
 complete -c labctl -n "__labctl_in_cmd provision debug" -l pxe-boot -d 'Boot installed system via PXE (kernel+initrd from network, root from NVMe)'
 
+# provision register options
+complete -c labctl -n "__labctl_in_cmd provision register" -l role -d 'Machine role' -xa 'vanilla worker infra labcontroller'
+complete -c labctl -n "__labctl_in_cmd provision register" -l ip -d 'Machine IP address' -x
+
 # provision logs options
 complete -c labctl -n "__labctl_in_cmd provision logs" -s f -l follow -d 'Follow log output in real-time'
 
@@ -179,6 +184,7 @@ complete -c labctl -n "__labctl_in_cmd app labcontroller status" -l user -d 'SSH
 complete -c labctl -n "__labctl_using_cmd app k3s" -a install -d 'Install k3s on a target machine (hostname, IP, or MAC)'
 complete -c labctl -n "__labctl_using_cmd app k3s" -a health -d 'Check k3s health (all hosts if no target given)'
 complete -c labctl -n "__labctl_using_cmd app k3s" -a list -d 'List installed machines and their k3s status'
+complete -c labctl -n "__labctl_using_cmd app k3s" -a kubeconfig -d 'Fetch kubeconfig from a target and merge into ~/.kube/config'
 
 # app k3s install options
 complete -c labctl -n "__labctl_in_cmd app k3s install" -l role -d 'k3s role: infra (server) or worker (agent)' -x
@@ -192,3 +198,8 @@ complete -c labctl -n "__labctl_in_cmd app k3s health" -l user -d 'SSH user' -x
 # app k3s list options
 complete -c labctl -n "__labctl_in_cmd app k3s list" -l user -d 'SSH user' -x
 
+# app k3s kubeconfig options
+complete -c labctl -n "__labctl_in_cmd app k3s kubeconfig" -l user -d 'SSH user' -x
+complete -c labctl -n "__labctl_in_cmd app k3s kubeconfig" -l context -d 'Context name (defaults to hostname)' -x
+complete -c labctl -n "__labctl_in_cmd app k3s kubeconfig" -l print -d 'Print kubeconfig to stdout instead of merging'
+