fix(docs): restore Agent main public-document truth

[AetherAI3]

Summary

Repair the post-merge README regression from #119 with a repository-owned copy of the existing visual.

Scope

• replace the GitHub user-attachment URL with assets/aether-agent-hero.png;
• include the deterministic 847x418, 474941-byte PNG in the npm allowlist;
• preserve public_docs_truth.test.ts and extend existing packaging validation to require the asset.

No SC-DEVICE functionality, release version, tag, or publication changes.

Evidence

• exact head: 8cfd7cc;
• source asset SHA-256: 8875c0ce3f42255d2309e684a1b7a34c1db920c64ae71240b4fdf8fb7c4622f7;
• existing hosted failure reproduced on both Ubuntu and Windows at 04ab613a: public_docs_truth rejected github.com/user-attachments/assets;
• git diff --check: pass;
• hosted CI required for final qualification because local dependency installation is unavailable in this environment.

This is the minimum corrective successor required because #119 was already merged and its source branch deleted before controller takeover.