Wi-Fi Provision for Core

wifi-provision hosts a WiFi access point with a captive portal so a phone or
laptop can configure a headless device's WiFi. It scans nearby networks via
wpasupplicant, hosts the portal AP with hostapd + dnsmasq (both bundled), and
joins the chosen network by writing netplan config and running netplan apply.

Designed for default Ubuntu Core (netplan + systemd-networkd + wpasupplicant) —
no NetworkManager required.