Add scripts/expose.sh: tailcat tunnel for remote access

tailcat (Tailscale data plane, no control plane) forwards the VM's
web UI and SSH ports to another machine via a stable saved-key
address; outbound-only, so it works through the nested slirp NAT.

README.md

@@ -54,6 +54,23 @@ scripts/run.sh
 - Ops SSH: `ssh -i configs/ssh/id_ed25519 -p 2222 marvin@localhost`
 - QEMU runs foreground; quit with `ctrl-a x`
 
+### Access from another machine (tailcat)
+
+`localhost` above is the machine running QEMU. To reach the VM from
+elsewhere, run `scripts/expose.sh` next to it — a tailcat tunnel
+(WireGuard over Tailscale's DERP relays, outbound-only, no account):
+
+```
+host $ scripts/expose.sh
+# 🐈 Server listening with saved key "default": tcpGFwWCCH...
+
+your $ tailcat forward tcpGFwWCCH... 8080:8080 2222:2222
+```
+
+Then browse `http://localhost:8080` / `ssh -p 2222 marvin@localhost`
+from *your* machine. The address is stable across VM restarts (saved
+key) and is a bearer secret — share it only privately.
+
 ### LLM backend
 
 `[models.tensorx]` in `configs/turnstone/config.toml` points at the