Regression: emitted nested visual artifacts are not fetchable by artifact get

[chubes4]

Problem

A fixture-matrix run published source, candidate, diff, and DOM snapshot artifacts in its structured summary with stable artifact IDs and public URLs. homeboy runs artifact get <run> <artifact-id> then returned:

artifact record not found

Only four top-level artifacts were listed as available. Two additional nested artifact retrieval attempts hung until the controller-side timeout. The same nested files remained downloadable through their public artifact URLs, so the data existed but the Homeboy retrieval contract could not resolve it.

Manual recovery required extracting URLs from large operator output and downloading each file separately with curl.

This regresses the nested-artifact indexing and retrieval contract previously addressed by #4036 and #4051.

Context: screenshot evidence for Automattic/blocks-engine#659.

Expected behavior

Every artifact ID emitted in a Homeboy-owned run summary is immediately listable and fetchable through homeboy runs artifacts and homeboy runs artifact get, including nested WP Codebox visual-comparison artifacts.

Acceptance criteria

• Nested visual comparison artifacts are indexed as first-class run artifacts.
• Every emitted artifact ID resolves through runs artifact get.
• Retrieval follows the public/object-store locator when the runner-local path is unavailable.
• Missing records fail quickly; retrieval never hangs waiting on an unreachable nested path.
• Coverage exercises source/candidate/diff screenshots and DOM snapshots from a nested fixture run.

AI assistance

• AI assistance: Yes
• Tool: OpenCode with OpenAI GPT-5.6 Sol
• Used for: Testing artifact retrieval, validating the public URL fallback, and comparing behavior with closed artifact-indexing issues.

[chubes4]

Fresh reproduction from Static Site Importer full-corpus run 7c356c92-9d00-4862-873c-0ccae65f6da3:

• The structured result emits nested source/candidate/diff refs for 15-saas and reports zero mismatch.
• homeboy runs artifacts 7c356c92-9d00-4862-873c-0ccae65f6da3 indexes only cli_run plus a partial set of editor-canvas artifacts. It does not index any visual_compare_15-saas_* artifact.
• The run-scoped artifact list contains no source, candidate, diff, visual-diff, DOM snapshot, or attribution bytes for that fixture, so runs artifact get cannot resolve reviewer evidence.
• The unscoped URLs embedded in the result (https://homeboy-artifacts-tunnel.dev.chubes.net/visual-compare/15-saas/...) are not stable run-scoped evidence. Existing operator-local images with the same fixture name have dimensions inconsistent with this run's recorded metric dimensions and therefore cannot be trusted as this run's evidence.

Impact: 15-saas is the sole nominal pass among 87 fixtures, but it cannot be reviewed or promoted because its exact screenshot evidence is not resolvable from the run. This blocks the solved-fixture gate even though summary metrics say it passed.

Reproduction:

homeboy runs evidence 7c356c92-9d00-4862-873c-0ccae65f6da3
homeboy runs artifacts 7c356c92-9d00-4862-873c-0ccae65f6da3

The matrix result is retained locally at /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/ssi-matrix-integrated-20260724/result.json; fixture 15-saas begins around its fixtures entry and includes all emitted nested refs.