docs(helia): add the content node to the architecture and target diagrams #75

Summary

Update the Helia integration overview once the node decision is made.

Estimated size: S

Proposal

• Update the architecture diagram, limitations (Section 12) and target architecture (Section 14) to include the Kubo node, and record the Boxo decision.

Acceptance criteria

• Diagrams render and describe the node as built.

Dependencies

• spike(content-node): what Kubo cannot do, and whether a custom Boxo service is justified #71

Part of #43. Ground rules for the content layer: no secrets in the repo, content-layer failures must never block or discard quantum execution, and tests and docs ship with every PR.