Bake mempalace-toolkit wrappers into the image

The scheduler templates in mempalace-toolkit's contrib/ assume
mempalace-session is available inside the container, but the image
never actually installed it. Users following the *-devbox scheduler
docs would silently lose the wrappers on every container recreate,
because the only way to get them was a post-hoc install.sh inside
the container — which lives in the ephemeral layer. The host-side
systemd timer would then fire, docker exec in, and hit
"mempalace-session: command not found".

Caught during runtime validation on 2026-04-30: host-side systemd
unit ran cleanly at 16:15 today, then the container was rebuilt
and recreated, and the wrappers were gone. The rebuild produced
an image that the scheduler template's own documented precondition
did not hold for.

Fix: new Dockerfile block clones mempalace-toolkit at build time
(depth-1) to /opt/mempalace-toolkit/, symlinks bin/mempalace-session
and bin/mempalace-docs into /usr/local/bin/, asserts both respond
to --help before the layer succeeds. Gated by
INSTALL_MEMPALACE_TOOLKIT=true (defaults on, depends on
INSTALL_MEMPALACE=true). Floated ref via MEMPALACE_TOOLKIT_REF=main
for auto-picking-up toolkit updates; override for reproducible
builds once the toolkit starts tagging releases.

Smoke test gains three assertions (mempalace-session --help,
mempalace-docs --help, symlink target check). Resolved-versions
preamble logs the toolkit git short-SHA alongside the other
floated components, so CI logs always record what got baked in.

README gains a Scheduled mining (mempalace-toolkit) subsection
and a build-args row. DOCKER_HUB.md regenerated; sync-check passes.

scripts/smoke-test.sh

@@ -71,6 +71,9 @@ docker run --rm --entrypoint="" "$IMAGE" sh -c '
   if command -v mempalace >/dev/null 2>&1; then
     printf "  %-15s %s\n" "mempalace"   "$(mempalace --version 2>&1 | head -1 || echo installed)"
   fi
+  if command -v mempalace-session >/dev/null 2>&1 && [ -d /opt/mempalace-toolkit ]; then
+    printf "  %-15s %s\n" "toolkit"     "$(git -C /opt/mempalace-toolkit rev-parse --short HEAD 2>/dev/null || echo installed)"
+  fi
 '
 echo
 echo "-- Core binaries --"
@@ -104,6 +107,16 @@ else
   echo "  - mempalace not installed (INSTALL_MEMPALACE=false)"
 fi
 
+# mempalace-toolkit wrappers: present unless built with INSTALL_MEMPALACE_TOOLKIT=false
+# Gated on mempalace presence — wrappers are useless without the CLI.
+if docker run --rm --entrypoint="" "$IMAGE" sh -c "command -v mempalace && command -v mempalace-session" >/dev/null 2>&1; then
+  run "mempalace-session (toolkit)" "mempalace-session --help | head -1"
+  run "mempalace-docs (toolkit)"    "mempalace-docs --help | head -1"
+  run "toolkit symlink target"      "test -L /usr/local/bin/mempalace-session && readlink /usr/local/bin/mempalace-session"
+elif docker run --rm --entrypoint="" "$IMAGE" sh -c "command -v mempalace" >/dev/null 2>&1; then
+  echo "  - mempalace-toolkit not installed (INSTALL_MEMPALACE_TOOLKIT=false)"
+fi
+
 # bun: only in the omos variant
 if [ "$VARIANT" = "omos" ]; then
   run "bun (omos)"            "bun --version"