fix(common): flatten Bluefin face icons into /usr/share/pixmaps/faces

[castrojo]

Closes #353

Problem

projectbluefin/common ships the Bluefin user-avatar art under
system_files/bluefin/usr/share/pixmaps/faces/bluefin/ (15 JPEGs, including the
dinosaur images). elements/bluefin/common.bst copies system_files/bluefin/usr/
verbatim, so those files land at /usr/share/pixmaps/faces/bluefin/ in the image
— confirmed by files/fakecap-manifest.tsv:657723-657737, which maps all 15 to
bluefin/common.bst.

GNOME's account-avatar chooser reads /usr/share/pixmaps/faces only, so the
images are present on disk but never offered to users. ublue-os/bluefin solves
this in build_files/base/05-override-install.sh (lines 29-32) by flattening
faces/bluefin/* over faces/ at build time; dakota has no equivalent step.

Change

• elements/bluefin/common.bst install-commands: move
  %{datadir}/pixmaps/faces/bluefin/* into %{datadir}/pixmaps/faces/ and
  remove the now-empty subdirectory, guarded by a -d test.
• Same element overlap-whitelist: add /usr/share/pixmaps/faces/*. The
  flattened filenames are identical to the GNOME OS defaults from
  core/gnome-control-center.bst (files/fakecap-manifest.tsv:657738-657752),
  so the overlap is intentional and must be declared.
• docs/skills/oci-layers.md: record the vendor-subdirectory pattern.

Validation

Not run — this environment has no container runtime (podman/docker absent),
so just bst, just validate, just build, just lint, and just boot-test
all cannot execute here. The change is unverified by build; please treat CI
validate + e2e as the gate.

What to check after a build:

just bst artifact list-contents oci/layers/bluefin.bst | grep pixmaps/faces
# expect 15 entries directly under usr/share/pixmaps/faces, no faces/bluefin/

Note on generated files: files/filemap.json and files/fakecap-manifest.tsv
still list the old faces/bluefin/* paths. Regenerating them requires
bst artifact list-contents against a populated local cache
(docs/skills/ci.md, "Generated Files"), which is not possible here. They are
refreshed by the monthly update-filemap.yml workflow; a maintainer can run
workflow_dispatch with force_regenerate after merge if an immediate refresh
is wanted. Stale entries are non-fatal — fakecap-restore skips paths that do
not exist (files/fakecap/fakecap-restore.c:67-73); the effect is only that the
15 avatars are unlabelled for chunkah layer assignment until regeneration.

• I am using an agent and I take responsibility for this PR

[github-actions]

This PR targets main — please retarget it to testing. All content PRs (features, fixes, BST changes) land on testing. The main branch is a release bookmark only.