WireGuard bring-up: host setup script, in-VM test tunnel, portable repo_root

wg-setup-host.sh sets up the tunnel on the backend host: keys generated
there and kept there, wg0.conf, wg-quick enabled, and the phone's config
printed as a scannable QR. Split tunnel (AllowedIPs is only the backend
subnet), single-address addressing per PLAN.md, and the three things it
can't do for you -- router UDP forward, DDNS, hairpin check -- spelled
out at the end.

test-wg-tunnel.sh stands up a real WireGuard tunnel between two network
namespaces inside one machine, so the production posture (bind wg0 and
nothing else) is testable with no router, phone, or internet exposure.
Verified: real handshake, server listening on 10.66.0.1:8443 only, and
an authorized request from inside the tunnel answering 200 over pinned
TLS -- the leaf's 10.66.0.1 SAN is what a phone will validate too.

repo_root() now resolves from the running executable before falling back
to the compiled-in path: the repo is shared host<->VM over virtiofs at
different absolute paths with a shared target/, so a binary built on one
side and run on the other looked for its config where nothing exists.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

server/wg-test.log

@@ -0,0 +1,4 @@
+2026-08-25T06:18:57.533304Z  INFO ai_server: config: /home/bob/host/repos/ai-app/config.json
+2026-08-25T06:18:57.537682Z  INFO ai_server: serving https://10.66.0.1:8443
+2026-08-25T06:20:43.470080Z  INFO ai_server: config: /home/bob/host/repos/ai-app/config.json
+2026-08-25T06:20:43.474488Z  INFO ai_server: serving https://10.66.0.1:8443