[TheGreatAxios]

Closes CL-11903

What changed

• The run-scoped write routes (POST /artifacts, /artifacts/binary, /artifacts/link-file, PATCH /artifacts/:id) return contentSha256 with id and version, so artifact_write does too. The value is the hash stored with the version; the tenant routes already returned it.
• CreatedWorkflowArtifact gains contentSha256: string | null (null only for versions written before hashes were stored).
• README documents the field; for link-file the digest covers the stored preview text, not the workspace file.
• artifact_write's description names the hash.

Verified

• e2e covers create, binary create, link-file and revise; nulling any return site fails a test.
• Sidecar tool output checked to carry the hash.
• bun run check; test:e2e 308 pass; test:coverage 339 pass with the floor met (throwaway Postgres).

[linear-code]

CL-11903

[corbits-triage]

Triage: Pass

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