Host-desktop: callback forward + xdg-open auto-open (flakes-ok62)


Status	Todo
Type	Task
Priority	normal
Tags	mcp
Parent	MCP OAuth: browser-open + callback forwarding (desktop + WSL) (flakes-mqe9)

Host-desktop (Case 1) side of flakes-mqe9. The human is at the host Linux desktop; incus exec into the container.

Callback (needed for Phase A too)

Host forward 127.0.0.1:3118 → 10.100.0.100:3118 (socat or systemd-socket-proxyd unit) so the desktop browser’s localhost:3118/callback reaches the container Caddy bridge → CC. Lives in a host (Linux) mixin.

Phase B — zero-click auto-open

Host systemd --user listener bound to the incus bridge IP that runs xdg-open <url> in the graphical session; authenticate with a shared secret.
Container $BROWSER (Phase A script) gains a host-desktop route: POST URL+secret to gateway:OPEN (gateway = ip -4 route show default).
Attach wrapper detects local desktop (WAYLAND_DISPLAY/DISPLAY, no SSH_CONNECTION) and injects BROWSER_ROUTE=host-desktop.

Bind the host listener to the incus bridge address (incus network get incusbr0 ipv4.address).