Add example for Cloudflare Workers Containers sandbox adapter

[astrobot-houston]

Originally proposed by @ghostwriternr in #334. Their description is reproduced verbatim below.

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Add a standalone examples/cloudflare-sandbox package showing how to run a
real Linux container sandbox on Cloudflare through Flue's cloudflareSandbox()
adapter.

examples/cloudflare demonstrates the @cloudflare/shell sandbox, which
exposes a JavaScript code tool over a Workspace rather than a real shell.
Its README already tells users who need bash, Linux tools, or mounted bucket
paths to reach for @cloudflare/sandbox (Workers Containers) instead — but
there was no runnable example of that path. This adds one as a separate
package so the Containers requirement stays opt-in.

The smoke workflow runs with model: false and drives the sandbox only
through harness calls, so it exercises the adapter boundary deterministically,
without depending on model output.

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Original implementation from #334 by @ghostwriternr