vendor: resync mempalace skill snapshot to skillset a12fe5e

scripts/vendor-mempalace-skill.sh, real refresh not --check: skillset
moved 6eb20af -> a12fe5e (mermaid-diagrams cutU normalisation, and the
from_agent identity rule this same release ships in RFC 003). --check
reported stale-but-truthful (exit 0, the sanctioned skip) but this
release's point is getting today's fixes live fleet-wide, and the base
rebuild is already forced by the entrypoint change and the floating
mempalace-toolkit ref moving -- so the incremental cost of also
bumping this pin is zero. Phrase canary in scripts/smoke-test.sh
unaffected: neither pinned phrase ('Provenance is stamped for you'
present, 'Attribute what you file yourself' absent) is in the section
that changed; both verified still correct in the new snapshot.

Dockerfile.variant

@@ -305,7 +305,7 @@ ARG MEMPALACE_TOOLKIT_REF=main
 # no ~67-minute base rebuild. (scripts/check-base-hash.sh scans only
 # Dockerfile.base, so no folding into the base hash is required — nor would
 # it be correct, since this ARG changes nothing about the base's contents.)
-ARG SKILLSET_SNAPSHOT_REF=6eb20af181f0147cb8c1377f6e36a6a47a68e8e5
+ARG SKILLSET_SNAPSHOT_REF=a12fe5ecc71e60feb24791e3e33571105f1afba7
 
 # Dockerfile.base sets description="pi-devbox — base image (variant-independent)"
 # and every variant INHERITS it, so both published images used to advertise