Hide & Show the Fusion Shell

Lets a Flows app hide the Fusion sidebar + topbar (the "shell") to use the full browser viewport, and reveal it again — without leaving the user stranded.

Requires @cognite/app-sdk's connectToHostApp() handshake already wired up, and @cognite/app-sdk >= 0.9.0 (verify with npm ls @cognite/app-sdk or check package.json — earlier versions don't expose setHideShell on HostAppAPI at all). If auth isn't wired up yet, run the setup-flows-auth skill first — but make sure it lands on the Apps API flow (app.json has "infra": "appsApi", deployed with npx @cognite/cli@latest apps deploy), not the Classic flow (DuneAuthProvider/useDune() from @cognite/dune). setHideShell only exists on the Apps API's HostAppAPI; Classic apps have no equivalent and are deployed to infrastructure @cognite/cli itself refuses to touch ("Legacy infrastructure is no longer supported"). If app.json is missing infra: "appsApi" or @cognite/dune shows up in package.json, stop and migrate to Apps API first — don't attempt this skill on a Classic app.