feat(ci): gate the vendored pi-extensions skill floor, and bake python3-yaml

Follows ecfd2fc, which refreshed the stale floor by hand. A one-off refresh
fixes the symptom; this makes the drift impossible to reintroduce silently.

scripts/check-skill-floor.sh compares the repo floor
(rootfs/usr/local/share/pi-devbox/skills/pi-extensions/) against the package
repo it is a snapshot of, wired in as a new `skill-floor` job in lint.yml.

DIRECTORY hash, not `sha256sum SKILL.md`, using the same tree_sha256 pipeline
Dockerfile.variant uses for skillset_snapshot_tree_sha256 and for the reason
already documented there: a file-only compare answers "did this one file
change", not "is this the same skill". Verified by NEGATIVE CONTROL rather
than asserted -- with SKILL.md left byte-identical and only
evaluate-extension-usage.py edited, the directory check fails (rc=1) where a
file-only compare would have passed. Seven behaviour tests, each with its
expected rc written down before running: in-sync via local dir (0), in-sync
via anonymous remote clone (0), missing --package-dir (2), bad argument (2),
content drift (1), the sibling-file case (1), and --warn-only over drift (0).

Exit codes 0 in sync / 1 drift / 2 cannot-run, matching scripts/lint-shell.sh:
a gate that cannot run must not pass, so an unreachable package repo is a red
2 and never a green tick. A ref with no skill/ is NOT drift -- that is the
documented fallback -- but it emits ::warning:: because it is precisely the
condition under which the floor ships.

Gating on another repo is normally a smell. It is proportionate here because
the check can only fire when skill/ itself changed, which is exactly when the
floor has gone stale; pi-extensions commits that leave skill/ alone cannot
turn this red. It also needs no secret: pi-extensions is anonymously clonable
(verified with `git ls-remote` and no credentials), so it cannot start failing
when a token expires.

Also bakes python3-yaml (552 KB, zero extra deps) into Dockerfile.base. This
is the shellcheck story repeating exactly: scripts/check-workflow-shell.sh --
the guard against the Gitea sh/dash footgun that broke resolve-versions
(ed49b8d) and promote-base-latest (b7197e8) -- hard-exits with "python3 yaml
module missing", so a gate this repo already owns could not be run locally by
anyone. lint.yml installing it explicitly in CI was the evidence. Found while
wiring the job above: the guard could not be run before pushing.

CHANGELOG Unreleased updated for both this and ecfd2fc, including an explicit
note on what is NOT fixed -- the silent-fallback half still has no manifest
flag recording which copy was served.

Verified locally with every gate this repo owns, all green: lint-shell.sh (15
files clean), check-workflow-shell.sh, check-base-hash.sh, actionlint 1.7.7
(pinned, same version as CI), hadolint 2.14.0, and the new check itself.

Dockerfile.base

@@ -162,6 +162,19 @@ ENV DEBIAN_FRONTEND=noninteractive
 #                  /dev/tcp does reachability checks with zero packages
 #                  (verified against gitea.egl.lan:3000). Recorded here so the
 #                  omission reads as a decision rather than an oversight.
+#   python3-yaml — PyYAML. Added 2026-09-10 for precisely the same reason as
+#                  shellcheck above: a gate this repo ALREADY OWNS could not be
+#                  run locally by anyone. scripts/check-workflow-shell.sh — the
+#                  guard that catches the "bash-only syntax under Gitea's default
+#                  sh/dash shell" footgun that broke resolve-versions (ed49b8d)
+#                  and promote-base-latest (b7197e8) — hard-exits with "ERROR:
+#                  python3 yaml module missing" without it. lint.yml installs it
+#                  explicitly in CI (`shellcheck python3-yaml`), which is itself
+#                  the evidence that the image lacked it. Measured 2026-09-10
+#                  while wiring the skill-floor job: the guard could not be run
+#                  before pushing — the same write → push → wait-for-CI loop that
+#                  shellcheck was baked to shorten. 552 KB, and pulls ZERO extra
+#                  packages under --no-install-recommends.
 RUN apt-get update && \
     apt-get upgrade -y --no-install-recommends && \
     apt-get install -y --no-install-recommends \
@@ -206,6 +219,7 @@ RUN apt-get update && \
     bind9-dnsutils \
     ldap-utils \
     xxd \
+    python3-yaml \
     && ln -s /usr/bin/fdfind /usr/local/bin/fd \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*