docs: add Open Design guide alongside Claude Design

[pftom]

Summary

Add a parallel handoff path for Open Design — the Apache-2.0, local-first, BYOK alternative to Claude Design that drives whichever coding-agent CLI the user already has on their PATH (Claude Code, Codex, Cursor, Gemini, OpenCode, Qwen, Copilot, Hermes, Kimi, Pi).

The integration story already exists on the Open Design side: their bundled motion-frames skill explicitly says "hand-off ready for HyperFrames". This PR closes the loop on the HyperFrames side so the route is discoverable from your docs and your README.

What's in the PR

Mirrors the existing Claude Design integration, file-for-file:

• README.md — one paragraph next to the Claude Design one, pointing at the new guide and explaining the drop-into-skills/SKILL.md install path.
• docs/guides/open-design.mdx — Mintlify page parallel to claude-design.mdx. Same structure: ``, comparison table, example prompts, limitations, handoff section.
• docs/guides/open-design-hyperframes.md — the SKILL.md-shaped instruction file users either drop into `skills/hyperframes-handoff/SKILL.md` inside Open Design (the daemon auto-discovers it on the next request and exposes it in the picker) or attach to chat as a one-shot.
• docs/docs.json — nav entry for the new page, slotted right after `guides/claude-design`.

The instruction file deliberately defers to `claude-design-hyperframes.md` as the canonical reference for skeleton catalogs, shader patterns, HDR, and audio-reactive animation — it stays focused on what Open Design's prompt stack needs at emission time (active-`DESIGN.md` binding, 5-dim self-critique gate, structural rules) so the two guides don't drift.

Why this is worth merging

• Zero-cost reach. Open Design users today have to write their own `SKILL.md` to hand off to HyperFrames. This PR turns that into "download one file, drop it into a folder, restart the daemon." That's the exact experience your Claude Design guide already provides — same install ergonomics, broader audience.
• Open-source surface area. Open Design ships 31 skills + 72 design systems + 10 CLI adapters under Apache-2.0. The route exists; documenting it from your side makes HyperFrames discoverable from theirs.
• No code, no build changes. Docs-only diff, no surface-area risk, no test impact.

Test plan

• Files added: docs/guides/open-design.mdx, docs/guides/open-design-hyperframes.md
• README paragraph reads naturally next to the Claude Design one
• Nav entry slotted after guides/claude-design in docs/docs.json
• Maintainer: render the Mintlify preview to confirm the new page formatting matches claude-design.mdx
• Maintainer (optional): suggest cross-linking from prompting.mdx's agent matrix