feat(studio): bundle studio-expose bridge + socat (opt-in STUDIO_EXPOSE)

pi-studio binds the container's 127.0.0.1, which a published Docker port
can't reach. Add a robust, portable bridge rather than a doc-only one-liner:

- Dockerfile.base: add socat (~1 MB, generally useful TCP relay).
- rootfs/usr/local/bin/studio-expose: socat TCP relay listening on the
  container's egress IPv4 (not 0.0.0.0 — that would EADDRINUSE against
  Studio's loopback listener) forwarding to 127.0.0.1:PORT on the SAME
  port, so Studio's printed token URL works verbatim. Robust egress-IP
  detection (hostname -I, loopback-filtered; ip route get fallback),
  --help, port validation, foreground.
- entrypoint-user.sh: opt-in STUDIO_EXPOSE=1 auto-starts the bridge in the
  background (studio variant only). Default OFF — Studio stays loopback-only
  (its secure default) unless explicitly opted in.
- README: 'Using pi-studio' now documents host-networking (A) and the
  studio-expose/STUDIO_EXPOSE bridge (B) with a security note; ssh -L for
  remote, mosh caveat retained.
- smoke-test: assert socat + studio-expose present (base-level).
- CHANGELOG/AGENTS updated.

No tag — stopping for review.

entrypoint-user.sh

@@ -121,6 +121,25 @@ if command -v pi &>/dev/null; then
   done
 fi
 
+# ── pi-studio: optional loopback bridge (opt-in) ──────────────────────
+# pi-studio binds its server to 127.0.0.1 inside the container, which a
+# published Docker port cannot reach. When STUDIO_EXPOSE is truthy (set in
+# compose), start the `studio-expose` socat bridge in the background so a
+# published port + `ssh -L` tunnel can reach Studio once the user runs
+# `/studio --port "$STUDIO_PORT"`. Default OFF — Studio stays loopback-only
+# (its secure default) unless explicitly opted in. Guarded on the studio
+# variant (/opt/pi-studio) so it is a no-op in the plain image.
+case "${STUDIO_EXPOSE:-}" in
+  1|true|TRUE|yes|on)
+    if [ -d /opt/pi-studio ] && command -v studio-expose &>/dev/null && command -v socat &>/dev/null; then
+      echo "STUDIO_EXPOSE set — starting studio-expose bridge on port ${STUDIO_PORT:-8765} (background)"
+      nohup studio-expose "${STUDIO_PORT:-8765}" >/tmp/studio-expose.log 2>&1 &
+    else
+      echo "STUDIO_EXPOSE set but studio-expose/socat/pi-studio unavailable — skipping bridge"
+    fi
+    ;;
+esac
+
 # ── Skillset: deploy skills/instructions from mounted skillset repo ──
 # When the skillset repo is mounted (at $HOME/skillset or /workspace/skillset),
 # run the deploy script to create relative symlinks for skills and instructions.