fix(ci): skip the BuildKit check whose warning embedded this Dockerfile in CI output

v1.10.1 (run 707) failed with ZERO failing steps: every step Success, `smoke`
printing "101 passed, 0 failed", `smoke-studio` "104 passed, 0 failed", both
jobs red. The clipboard fix from f3b3748 worked exactly as predicted; this is a
second, independent defect that run 704 had masked.

CHAIN, measured end to end rather than reasoned:

`ARG BASE_IMAGE` has no default on purpose (the two-phase build always supplies
it), which trips BuildKit's InvalidDefaultArgInFrom check. buildx attaches that
warning's source context to the build metadata as
buildx.build.warnings[].sourceInfo.data — the ENTIRE Dockerfile, base64, on ONE
line. docker/build-push-action writes that metadata to $GITHUB_OUTPUT as a
`name<<ghadelimiter_<uuid>` heredoc. Gitea's act_runner truncates any single
line at exactly 65536 chars, so the closing delimiter was cut off:

  invalid format delimiter 'ghadelimiter_...' not found before end of file

and the runner failed the job while attributing it to no step at all.

  v1.9.4  run 695 SUCCESS: longest metadata line 56355, 0 delimiter errors
  v1.10.0 run 704 failed : longest metadata line 65536, 1 delimiter error
  v1.10.1 run 707 failed : longest metadata line 65536, 1 delimiter error

65536 = 2^16: cut AT the cap, not merely long. Independent route via file size
rather than log parsing: 42251 B at v1.9.4 (base64 56335, matching the log) vs
50034 B now (base64 66712, truncated). The cap corresponds to a 49152 B
Dockerfile, so this cycle's comment growth crossed it by 882 B.

Located by asking which top-level metadata key CONTAINS the base64, instead of
assuming: it is buildx.build.warnings -> sourceInfo -> data in BOTH runs.

THIS IS THE SECOND FIX FOR THIS BUG. The first, `provenance: false` on the two
smoke build steps, was committed as 784fad7 and is REVERTED here: a real buildx
on another host showed provenance metadata present in both modes with a longest
string of 71 chars, i.e. the base64 was never in provenance. Shipping it would
have left the release broken a third time while looking like a fix.

Also rejected, each measured: floating action tags moving (act action-bundle
hashes byte-identical between runs 695 and 707, all four) and the build-check
annotation text changing (byte-identical).

FIX: `# check=skip=InvalidDefaultArgInFrom` as the FIRST line of
Dockerfile.variant — BuildKit parses `# check=` only before any other line, so
placement is load-bearing. No honest default exists for BASE_IMAGE: `scratch`
would satisfy the linter while being a lie, and would convert today's instant
"invalid reference format" into a failure deep in the build. Verified against
the actual edited file with `docker buildx build --check`: "Check complete, no
warnings found." Zero warnings means no sourceInfo, so the longest metadata line
drops 65536 -> ~1936 and the file's SIZE stops gating CI.

GUARD: scripts/check-dockerfile-directives.sh, wired into BOTH lint.yml and
docker-publish.yml's lint-gate, because a check that gates only `push` lets a
tag regress — the adoption slip that let doc-drift land 27 h after the v1.9.3
tag. It also fails if ARG BASE_IMAGE gains a default, so the directive cannot
rot into guarding a check that can no longer fire. Truth table, exit codes:
present 0, removed 1, demoted to line 2 → 1, ARG defaulted 1, file missing 2
(a gate that cannot run must not pass).

Release renamed v1.10.1 -> v1.10.2 with both failed tags left standing as
tombstones. Docs swept again (README "since" marker, Dockerfile decision
comments) because CI reads them from the TAG.

Gates: doc-drift 23 OK / 0 DRIFT; base-hash, workflow-shell, skill-floor,
lint-shell (17 files now), dockerfile-directives all rc=0.

Dockerfile.variant

@@ -1,3 +1,37 @@
+# check=skip=InvalidDefaultArgInFrom
+#
+# ^ MUST stay the FIRST line of this file, and it is load-bearing for CI, not
+# style. BuildKit parses `# check=` only before any other line, so moving it
+# below the title comment silently disables it.
+#
+# Why it exists: `ARG BASE_IMAGE` (below) deliberately has NO default — this
+# file is only ever built by the two-phase CI with --build-arg BASE_IMAGE=
+# <image>:base-<hash>. BuildKit's InvalidDefaultArgInFrom check flags that as
+# "default value for global ARG results in an empty or invalid base image
+# name". There is no honest default to give it: `scratch` would satisfy the
+# linter while being a lie (nothing here can build FROM scratch), and it would
+# convert today's instant "invalid reference format" into a failure deep in the
+# build. So the check is skipped by name, with the reason written down.
+#
+# What the warning actually COST, which is why this is not cosmetic: buildx
+# attaches the warning's source context to the build metadata as
+# buildx.build.warnings[].sourceInfo.data — the ENTIRE Dockerfile, base64, on
+# ONE line. docker/build-push-action writes that metadata to $GITHUB_OUTPUT as
+# a `name<<ghadelimiter_<uuid>` heredoc, and Gitea's act_runner truncates any
+# single line at exactly 65536 chars. Once base64(this file) crossed 64 KiB the
+# closing delimiter was cut off, and the runner failed the job with
+#   invalid format delimiter 'ghadelimiter_...' not found before end of file
+# and NO failing step: every step green, smoke suite "0 failed", job red.
+# Measured: 42251 B at v1.9.4 -> base64 56355 (fine); 50034 B at v1.10.1 ->
+# base64 66712, truncated to exactly 65536. The cap corresponds to a 49152 B
+# Dockerfile, so this release's comment growth crossed it by 882 B. It killed
+# v1.10.0 (run 704, where a stale clipboard assertion hid it) and v1.10.1
+# (run 707). With zero warnings there is no sourceInfo, so the longest metadata
+# line drops from 65536 to ~1936 chars and the Dockerfile's SIZE stops being
+# coupled to CI passing at all.
+#
+# If this ever recurs — job red, zero failing steps, suite reporting 0 failed —
+# grep the job log for `ghadelimiter` first.
 # pi-devbox — variant image
 #
 # FROMs a base-<hash> image produced by Dockerfile.base and adds only
@@ -154,11 +188,11 @@ ARG USER_NAME=developer
 # (e7d77dc -> 1529e14 -> 731c3d4 in the nine days to 2026-10-01), so waiting for
 # a tag means holding pi indefinitely. A full SHA keeps the one property
 # `master` does not have: rebuilding this tag later produces the SAME image.
-# v1.9.5 SKIPPED 0.99.x and 1.0.0 for lack of evidence. v1.10.1 went and got the
+# v1.9.5 SKIPPED 0.99.x and 1.0.0 for lack of evidence. v1.10.2 went and got the
 # evidence instead of waiting for obsmem to mention a version, because "no issue
 # names 1.0" is an absence of a statement, not a measurement.
 #
-# v1.10.1: 0.87.1 -> 1.0.0. MEASURED 2026-10-02 against the published npm
+# v1.10.2: 0.87.1 -> 1.0.0. MEASURED 2026-10-02 against the published npm
 # tarballs for 0.87.1 and 1.0.0, unpacked side by side:
 #   1. NO `### Breaking Changes` SECTION IN 1.0.0 AT ALL. The changelog's
 #      breaking sections belong to 0.87.0, 0.86.0, 0.84.3, 0.84.0, 0.83.0,
@@ -273,7 +307,7 @@ ARG PI_ATELIER_REPO=https://github.com/michaelmjhhhh/pi-atelier.git
 # pi >=0.84.0, so it still spans the pinned 0.85.1. 13 commits v0.10.1..v0.10.3,
 # all under src/ tests/ docs/ scripts/ plus metadata; no entry-point move.
 #
-# v1.10.1: v0.10.3 -> v0.13.0, closing the two-minor gap v1.9.5 flagged as the
+# v1.10.2: v0.10.3 -> v0.13.0, closing the two-minor gap v1.9.5 flagged as the
 # residual risk of the pi bump. peerDependencies are UNCHANGED at pi >=0.84.0
 # across v0.10.3, v0.12.1 and v0.13.0 — still a FLOOR, so still not evidence of
 # anything; the floor is satisfied by 1.0.0 either way. What was actually