fix(ci): generate the SBOM with Syft instead of a BuildKit attestation

[grinidx]

What broke

The weekly release run on 16 Aug failed on both architectures at Push by digest (run 31924690964):

failed to solve: /tmp/buildkit-mount1937992494/sbom.spdx.json exceeds 41943040 bytes

No commit here since 9 Aug, and that morning's release was green.

Root cause

docker/setup-buildx-action boots the floating moby/buildkit:buildx-stable-1 tag:

run	BuildKit	result
9 Aug	v0.31.2	green
16 Aug	v0.32.2	red

Same syft scanner in both (buildkit-syft-scanner:stable-1, v1.11.0), so the SBOM itself is unchanged.

v0.32 replaced the plain json.NewDecoder(f) in exporter/attestation/unbundle.go with a new decodeStatement that wraps the reader in io.LimitedReader{N: maxAttestationBytes + 1}. maxAttestationBytes is 40 MiB, exactly the 41,943,040 in the error. The constant has existed since 2022; v0.32 is the first version to enforce it on that path (moby/buildkit@9cdf6429).

Our SBOM has been over that line for months. Off the last good release, v2026.08.09:

arch	SBOM bytes
linux/amd64	52,276,285
linux/arm64	51,944,931

Breakdown: files 25.9 MB (50,656 entries), relationships 15.8 MB, packages 10.6 MB (6,838). Inherent to a container this broad, and there is no supported way to trim it - buildkit-syft-scanner reads only BUILDKIT_SCAN_DESTINATION, SOURCE and SOURCE_EXTRAS.

The change

Stop routing the SBOM through a mechanism with a ceiling we do not control.

• build.yml: drop sbom: true from Push by digest; add a Syft scan of the image already built, loaded and tested, uploaded as a per-arch artifact. provenance: mode=max is a few KB and is untouched.
• release.yml: attach the two SPDX documents instead of extracting one from the image. The GHCR login there existed only for the extraction, so it goes too.

The scan runs on every build, publishing or not, so this PR's own CI proves it works; only the upload is gated on publish.

What consumers see

• Release assets become sbom-amd64.spdx.json.gz and sbom-arm64.spdx.json.gz, each a plain SPDX 2.3 document, replacing the single sbom.json.gz that wrapped both architectures in a bespoke buildx envelope. The old notes called that CycloneDX; it was SPDX then too.
• The image no longer carries an SBOM attestation, so imagetools inspect --format '{{ json .SBOM }}' no longer returns one. README, SECURITY and ARCHITECTURE now point at the release assets.

Rejected

• Pin moby/buildkit:v0.31.2. One line, green today, but gives up a hardening release and only defers this.
• Shrink the SBOM. Would need a forked generator image to reach syft's config.