docs: redesign Hookbuilder public presentation

[0xprogrammable]

Summary

• rebuild the public README around a clear install, use, contribution, and evidence flow
• replace the repository cover with the Programmable Night Garden ant-collective composition
• add a 1280 x 640 repository social-preview asset and document its publication boundary
• keep v0.4.0 explicit as the latest published release and v0.5.1 explicit as development source

Scope

Presentation only: README.md and assets/**. No Skill, plugin, MCP, compiler, contract, security rule, test, or release-gate path changed.

Validation

• npm test — REPOSITORY_VALID; all seven repository gates passed
• npm run plugin:check — 645 files, source-byte verification passed, digest unchanged
• gh skill publish --dry-run — passed
• rendered README QA at 1440 px, 390 px, and 320 px; no horizontal page overflow or console errors
• cover and social preview decode correctly and remain below the repository 1 MB limit