feat(studio): add :latest-studio variant (PR-3)

Bundle pi-studio (omaclaren/pi-studio) as a new -studio image variant:
browser prompt editor, KaTeX/Mermaid preview, tmux-backed literate REPLs,
/studio command + studio_* agent tools.

- Dockerfile.variant: INSTALL_STUDIO + PI_STUDIO_REPO/REF args; vendor
  pi-studio to /opt/pi-studio (no build step — prebuilt client in git;
  npm install --omit=dev for 3 prod deps). STUDIO_PORT=8765 advisory.
- entrypoint-user.sh: register /opt/pi-studio via the existing pi install
  local-path loop (auto-skips in non-studio variant).
- smoke-test.sh: auto-detected studio assertions (clone + prebuilt client
  + pi install registration).
- CI: resolve PI_STUDIO_REF to a SHA; independent smoke-studio +
  build-variant-studio jobs that gate ONLY the -studio tags, so a studio
  failure never blocks the core :latest release.
- README: 'Using pi-studio' section documenting the container access
  reality — pi-studio hard-binds 127.0.0.1 (index.ts .listen(port,
  '127.0.0.1'), no --host flag), so -p publish alone can't reach it.
  Documents host-networking and loopback-bridge paths, the remote ssh -L
  forward, and the mosh caveat (no port forwarding; run parallel ssh -L).
- CHANGELOG/AGENTS/DOCKER_HUB updated. Will tag as v1.1.0 (minor).

No tag created — stopping for review.

scripts/smoke-test.sh

@@ -15,6 +15,8 @@
 #   - mempalace bridge symlink present
 #   - settings.json bootstrapped
 #   - pi-fork + pi-observational-memory registered via `pi install`
+#   - (studio variant only, auto-detected) pi-studio cloned + prebuilt
+#     client bundle present + registered via `pi install`
 #   - image size within threshold
 
 set -euo pipefail
@@ -95,6 +97,20 @@ run "pi-fork clone + node_modules" \
 run "pi-observational-memory clone + node_modules" \
     "test -f /opt/pi-observational-memory/package.json && test -d /opt/pi-observational-memory/node_modules"
 
+# pi-studio is present only in the :latest-studio variant. Auto-detect by
+# probing /opt/pi-studio so this one script covers both variants.
+if docker run --rm --entrypoint="" "$IMAGE" sh -c 'test -d /opt/pi-studio' >/dev/null 2>&1; then
+  STUDIO_VARIANT=1
+  echo "  ℹ️   pi-studio detected — running studio assertions"
+  run "pi-studio clone + node_modules" \
+      "test -f /opt/pi-studio/package.json && test -d /opt/pi-studio/node_modules"
+  run "pi-studio prebuilt client bundle" \
+      "test -f /opt/pi-studio/client/studio-client.js"
+else
+  STUDIO_VARIANT=0
+  echo "  ℹ️   pi-studio not present (non-studio variant) — skipping studio clone checks"
+fi
+
 # ── Runtime deployment (needs entrypoint to run) ──────────────────────
 echo ""
 echo "── Runtime deployment ──"
@@ -150,6 +166,20 @@ done
 exec_test "pi-fork registered (fork tool)"         'grep -q pi-fork $HOME/.pi/agent/settings.json && echo ok'
 exec_test "pi-observational-memory registered (recall tool)" 'grep -q pi-observational-memory $HOME/.pi/agent/settings.json && echo ok'
 
+# pi-studio registration (studio variant only) — registered by the same
+# entrypoint-user.sh local-path install loop as fork/obsmem.
+if [ "${STUDIO_VARIANT:-0}" = "1" ]; then
+  for i in $(seq 1 15); do
+    if docker exec "$CID" grep -q pi-studio \
+         /home/developer/.pi/agent/settings.json 2>/dev/null; then
+      break
+    fi
+    sleep 1
+  done
+  exec_test "pi-studio registered (/studio command + studio_* tools)" \
+    'grep -q pi-studio $HOME/.pi/agent/settings.json && echo ok'
+fi
+
 # ── /tmp/sshcm directory created by entrypoint ────────────────────────
 exec_test "/tmp/sshcm dir mode 700 (ssh ControlMaster)" \
   'test -d /tmp/sshcm && [ "$(stat -c %a /tmp/sshcm)" = "700" ] && echo ok'