skills: record the vendored snapshot's provenance, and report which copy wins

Found while verifying v1.8.7 from inside a fresh container: the baked mempalace
snapshot is read by no host on this fleet. devbox-skill-reconcile repoints
~/.agents/skills/mempalace at the mounted live clone (the v1.8.5 fix working as
designed), and all four compose stacks mount a workspace containing the
skillset. So the phrase canary that blocked v1.8.7's first tag polices a file
nobody opens, while the drift that could actually mislead an agent — a git pull
nobody ran in /workspace/skillset — was invisible from inside the container and
is invisible to CI by construction.

Record provenance instead of policing it, and move the check to where the
skillset actually is:

- Dockerfile.variant: ARG SKILLSET_SNAPSHOT_REF (the claim) + a sha256 of the
  shipped bytes measured in the manifest layer (the fact), as manifest siblings
  rather than components{} members, plus an OCI label. An ARG default, not a
  CI-resolved output: no credential for the private skillset, no change at any
  of the four variant build call sites, and a local docker build records what CI
  does. Variant-only, so no base rebuild — check-base-hash.sh scans
  Dockerfile.base alone, verified by running it.
- pi-devbox-version: a skills: section naming baked vs live <repo> @ <sha> per
  vendored skill, and for mempalace whether the live copy is identical to the
  baked fingerprint, at the same commit with uncommitted edits, or divergent.
  entrypoint-user.sh passes the new --no-skills, because the banner prints
  before the links exist and long before the reconcile runs.
- scripts/vendor-mempalace-skill.sh: refresh the file and rewrite the ref
  together (a cp without an ARG bump makes the manifest lie, which is worse than
  anonymity); --check verifies the claim against a real clone.
- 5 new smoke assertions (78 -> 83), mutation-tested through the real sh -c
  path: 6 fabricated manifests, where a well-formed hash of the wrong file
  proves the two manifest assertions are not redundant; the all-baked reporting
  test verified to FAIL against a live-skillset environment.

Reviewed mid-flight by pi@emb-7kj4vr4g over the logstream (correlation
skillset-vendor-drift), which retracted its own earlier recommendation of a
build-time byte-compare against skillset HEAD and supplied the better framing:
the invariant is NON-CONTRADICTION, not currency. Byte parity on a fallback
would have cost a resync commit plus a ~67-min base rebuild for each of the four
skillset commits pushed in one evening. Its warning also found a real bug here:
the script now CONSTRUCTS the snapshot from `git show HEAD:<path>` instead of
copying the working tree, because a clean `git diff` says nothing about an
untracked file — the one input the first draft would have recorded a false ref
for. Tested: untracked, unstaged and staged-but-uncommitted all refuse, atomically.

Also fixes three stale in-repo markers of the same class the canary belongs to
(true when written, silently false at release): two dangling "Unreleased"
pointers and a typst line still marked Unreleased five releases after v1.4.0.

rootfs/usr/local/bin/pi-devbox-version

@@ -14,6 +14,8 @@
 #   pi-devbox-version           human-readable summary (default)
 #   pi-devbox-version --json    raw manifest JSON (for scripting)
 #   pi-devbox-version --quiet   one-line "release_tag (source_revision)" form
+#   pi-devbox-version --no-skills   skip the skill-source section (used at
+#                               container start, where it would be premature)
 #
 # EXIT STATUS
 #   0 on success. 1 if the manifest is missing (e.g. an image built before
@@ -24,12 +26,14 @@ set -euo pipefail
 
 MANIFEST=/etc/pi-devbox/build-manifest.json
 MODE="human"
+SHOW_SKILLS="yes"
 
 case "${1:-}" in
   --json)  MODE="json" ;;
   --quiet|-q) MODE="quiet" ;;
+  --no-skills) SHOW_SKILLS="no" ;;
   --help|-h)
-    sed -n '2,20p' "$0" | sed 's/^# \?//'
+    sed -n '2,22p' "$0" | sed 's/^# \?//'
     exit 0
     ;;
 esac
@@ -105,3 +109,92 @@ fi
 
 printf '  components:\n'
 jq -r '.components | to_entries[] | select(.value != null) | "    \(.key): \(.value[0:12])"' "$MANIFEST"
+
+# ── Which copy of each vendored skill is actually being read? ─────────
+# The image bakes fallback skills under /usr/local/share/pi-devbox/skills/,
+# but for skills the skillset repo OWNS (skillset-owned.txt) a mounted live
+# clone takes over at container start via devbox-skill-reconcile. Nothing
+# reported which copy won, so a stale baked snapshot and a current live clone
+# looked identical from inside — and on this fleet the baked mempalace copy is
+# read by NOBODY (all four compose stacks mount a workspace containing the
+# skillset), which is exactly the sort of fact that should be visible rather
+# than reasoned about. Same "drift detected" shape as the pi/palace lines
+# above: what is live, annotated with what was baked, when they disagree.
+#
+# Skipped with --no-skills at container start (entrypoint-user.sh calls this
+# FIRST, before the baked links exist and long before the skillset deploy and
+# reconcile run last), because a section that is accurate only after boot
+# finishes is worse than no section at all.
+BAKED_SKILLS=/usr/local/share/pi-devbox/skills
+SKILLS_DIR="${HOME:-/home/developer}/.agents/skills"
+
+if [ "$SHOW_SKILLS" = "yes" ] && [ -d "$BAKED_SKILLS" ] && [ -d "$SKILLS_DIR" ]; then
+  # Recorded provenance of the vendored mempalace snapshot (absent on images
+  # built before this existed — `// empty` so a JSON null never prints as the
+  # 4-char string "null", the same trap noted for mempalace_version above).
+  snap_ref=$(jq -r '.skillset_snapshot_ref // empty' "$MANIFEST")
+  snap_sha=$(jq -r '.skillset_snapshot_sha256 // empty' "$MANIFEST")
+
+  # Iterate the baked tree rather than a hardcoded name list, so vendoring a
+  # fourth skill needs no edit here. The header prints only if the tree is
+  # non-empty, so this can never emit a dangling "skills:" label.
+  _printed_header="no"
+  for _dir in "$BAKED_SKILLS"/*/; do
+    [ -d "$_dir" ] || continue
+    if [ "$_printed_header" = "no" ]; then
+      printf '  skills:\n'
+      _printed_header="yes"
+    fi
+    _name=$(basename "$_dir")
+    _link="$SKILLS_DIR/$_name"
+
+    if [ ! -e "$_link" ]; then
+      printf '    %-22s not linked\n' "$_name"
+      continue
+    fi
+
+    _target=$(readlink -f "$_link" 2>/dev/null || echo "$_link")
+    case "$_target" in
+      "$BAKED_SKILLS"/*|"$BAKED_SKILLS")
+        printf '    %-22s baked\n' "$_name"
+        continue
+        ;;
+    esac
+
+    # Outside the baked tree: a mounted skillset clone, or a user override.
+    # The link target is <repo>/skills/<name>, so the repo root is two up.
+    # Everything here is guarded: this script runs on the container-start path
+    # and must never fail, and `set -e` is in force.
+    _root=$(cd "$_target/../.." 2>/dev/null && pwd) || _root=""
+    _head=""
+    if [ -n "$_root" ]; then
+      _head=$(git -C "$_root" rev-parse HEAD 2>/dev/null || echo "")
+    fi
+    _where="live ${_root:-$_target}"
+    [ -n "$_head" ] && _where="$_where @ ${_head:0:7}"
+
+    # For the one skill whose baked fingerprint we recorded, say plainly
+    # whether the live copy differs from what shipped. This is the check CI
+    # cannot perform (the skillset is private) and the container can, free.
+    _live_sha=""
+    if [ -n "$snap_sha" ] && [ "$_name" = "mempalace" ] && [ -f "$_target/SKILL.md" ]; then
+      _live_sha=$(sha256sum "$_target/SKILL.md" 2>/dev/null | cut -d' ' -f1 || echo "")
+    fi
+    if [ -z "$_live_sha" ]; then
+      printf '    %-22s %s\n' "$_name" "$_where"
+    elif [ "$_live_sha" = "$snap_sha" ]; then
+      printf '    %-22s %s  (identical to baked snapshot)\n' "$_name" "$_where"
+    elif [ -n "$_head" ] && [ "$_head" = "$snap_ref" ]; then
+      # Same commit, different bytes — i.e. uncommitted edits in the live
+      # checkout. Distinguished from plain drift because otherwise the line
+      # reads as a self-contradiction ("@ c04cd15 ... baked snapshot c04cd15
+      # — live copy differs") and a reader would suspect the tool, not the
+      # working tree.
+      printf '    %-22s %s  \033[33m(baked snapshot %s + uncommitted edits)\033[0m\n' \
+        "$_name" "$_where" "${snap_ref:0:7}"
+    else
+      printf '    %-22s %s  \033[33m(baked snapshot %s — live copy differs)\033[0m\n' \
+        "$_name" "$_where" "${snap_ref:0:7}"
+    fi
+  done
+fi