feat: add AgentsKit sandbox example

[EmersonBraun]

What changed

• add a TypeScript example using the native AgentsKit E2B sandbox backend
• run JavaScript and Python with explicit per-execution timeouts
• print structured stdout, stderr, exit code, and duration
• guarantee sandbox disposal with finally
• add the example to the cookbook index

Why

This gives E2B users a small framework integration that does not require a model provider and demonstrates lifecycle and failure-handling patterns around the Code Interpreter SDK.

Validation

• npm ci
• npm run check
• npm audit --omit=dev
• live JavaScript execution: exit code 0, {"sum":29}
• live Python execution: exit code 0, {'mean': 7.25}
• git diff --check

The example uses the published @agentskit/sandbox@0.6.1 release. No credentials are included in the repository.

[EmersonBraun]

Hi — would this cookbook example still be useful here? The PR is mergeable, the CLA check passes, and the example covers both successful execution and failure mapping with the published @agentskit/sandbox package. I'm happy to align it with any newer cookbook conventions.