bug: SSH key entered in User step never attached to UserConfig — install produces unreachable host

[castrojo]

Summary

SSH keys entered in the TUI User step are written to Config.SSHKeys (top-level) but never to Config.Users[0].SSHKeys. The Butane template only renders per-user SSH keys when Users is non-empty, so the generated Ignition config produces a user account with no SSH keys.

Severity: Critical

This silently produces an installed system that the user cannot log into.

Root Cause

In internal/tui/tui.go applyFields(), the User step handler writes the SSH key to the wrong place:

case "ssh_key":
    if f.value != "" {
        cfg.SSHKeys = []string{f.value}   // ← top-level SSHKeys
    }

But it also creates a UserConfig (or updates Users[0].Username) without copying the SSH key into Users[0].SSHKeys.

Meanwhile, the Butane template in internal/ignition/ignition.go only uses per-user keys when Users is populated:

{{- range .Users}}
    - name: "{{.Username}}"
    ...
    {{- if .SSHKeys}}         ← this is user.SSHKeys, which is empty
      ssh_authorized_keys:
    {{- range .SSHKeys}}

The top-level SSHKeys is only used in the else branch for the default core user (when Users is empty). Since the TUI always creates a UserConfig, that branch is never reached.

Steps to Reproduce

1. Start knuckle
2. Navigate to the User step
3. Enter a username (e.g., admin) and an SSH key (e.g., ssh-ed25519 AAAA... user@host)
4. Continue to Review → Install
5. Inspect the generated Butane YAML

Expected

The SSH key appears under the user's ssh_authorized_keys in the Butane output.

Actual

The user entry has no ssh_authorized_keys block. The SSH key is stored in Config.SSHKeys but never rendered.

Fix

In applyFields(), when processing the ssh_key field, also write to cfg.Users[0].SSHKeys:

case "ssh_key":
    if f.value != "" {
        cfg.SSHKeys = []string{f.value}
        if len(cfg.Users) > 0 {
            cfg.Users[0].SSHKeys = []string{f.value}
        }
    }