feat(shell): preload shared contracts and Electron boundary

[PerishCode]

Outcome

Adds the shared @open-design/shell launch-context and update persistence contracts, then reserves shells/electron as the Electron-specific adapter workspace.

Transparency boundary

• The shared Shell package is buildable and tested but has no current consumer.
• shells/electron contains only package ownership and boundary documentation.
• The Electron workspace has no scripts, exports, source, packaging, or release entrypoint.
• No existing application or workflow changes behavior.

Validation

• pnpm install --frozen-lockfile including all repository postinstall builds
• Shared Shell contracts: 9 tests + typecheck
• Electron boundary is dependency-free and inert

[github-actions]

🚀 Landing page preview

This PR is deployed to a Cloudflare Pages preview — not staging or production:

• Stable alias: https://pr-7063.open-design-landing-staging.pages.dev
• This build: https://efb903b1.open-design-landing-staging.pages.dev
  Updated for commit 46f97b6.

[PerishCode]

Closing the stacked review PR to avoid implying a non-main landing path. The branch is intentionally retained as a preload boundary for future integration.

[lefarcen]

Makes sense — thanks for clarifying the intent here. Keeping the branch around as a preload boundary for a later mainline integration sounds good.