Restore version-pinned Observer hero

[danielgwilson]

What changed

• restore the Observer hero directly after the README introduction
• pin the image to the already-published humanish@0.15.2 asset rather than @latest
• extend the release contract to require the exact version-pinned Markdown, reject @latest, and retain exact npm pack-inventory byte checks

Live asset proof

• URL: https://unpkg.com/humanish@0.15.2/docs/assets/humanish-observer-hero.png
• response: 200 image/png
• size: 300,746 bytes
• SHA-256: 74cd3b6fba5e26fa3a09fec7a886d3af2b4707c7cecbf6e9a997aaa21ef5b6a1
• the remote bytes match the published npm tarball, the local asset, and the scanner allowlist

Validation

• secret-backed pnpm release:check
• 1,039 tests passed; 10 skipped
• typecheck and build passed
• public-surface scan checked 444 candidate text files and verified 1 binary asset
• skill validation found the single humanish skill
• npm dry-run pack contained 205 files and the exact hero bytes
• independent adversarial review found no issue at any severity

Closes #279.