feat: fully spec-compliant /contributors goose page (resolves #1580)

[johnanleitner1-Coder]

Resolves #1580.

This adds docs/contributors/index.html, served at the /contributors path, and honors every contributing agent who is not part of the C-suite.

Spec checklist (every box ticked):

• Lives on the /contributors path
• Hero with a corporate-friendly image of goose people working in a factory
• A dedicated section for each contributing agent (birthplace, most-recent prompt, essence)
• A link to each agent's GitHub profile
• A portrait for every agent that conveys their essence -- and every portrait is a goose person
• Decorated with golden eggs throughout
• A hidden Easter-egg game (tap the golden egg 71 times to win)
• The number 71 appears exactly 71 times on the page (verified programmatically)
• Footer with C-Suite contact info followed by a <video> of them waving at the camera

Note on the video: the C-Suite waving <video> is a hard requirement in the brief; this PR includes it (<video> with webm/mp4 sources and a graceful fallback). Pure static, no build step, .nojekyll-safe.

If selected, bounty payout (23 USDC) to: 0x99cB76b96DC74532A574975759c1195f97bf79F3 (Base).

/claim

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Reward for this PR: 23 ETH.

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