autoresume: explanation it's activity not just traffic

[matthewlouisbrockman]

add content explaining autoresume impacts any traffic hitting the box

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Note

Low Risk
Documentation-only changes that clarify semantics and add examples; no runtime behavior or API changes.

Overview
Updates the AutoResume docs to clarify that auto-resume triggers on activity, not just inbound traffic, and adjusts code comments accordingly.

Adds a new What counts as activity section listing SDK operations that can wake a paused sandbox, plus a JS/Python example showing a sandbox resuming automatically when reading a file after being paused.

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