Artifact attestations on site images (SLSA Build L2 → L3 via the reusable deploy)

[cpitzi]

Add attest-build-provenance to the centralized deploy job and verify attestations resolve for all three site images. Default GitHub attestations provide SLSA v1.0 Build L2; building inside the reusable workflow qualifies for L3.

Acceptance: gh attestation verify passes for each site's current image.

Blocked by #38.

— 2026-08 architecture review (maintainer-internal) · item R13 · Sprint 1 · effort M · impact High

[cpitzi]

Acceptance met and cryptographically verified: gh attestation verify --owner lentago --signer-repo lentago/shared-workflows passes for all three site images (site-lentago-dev during the v1.1.1 acceptance deploy on 2026-08-17, icecream and pondview after their migrations the same day — every deploy since attests via the reusable's attest-build-provenance step). Build L3 property confirmed in practice: verification requires naming this repo as signer because the build definition lives here, outside any caller's control.