agent-sandbox — pluggable execution sandbox for tool calls.
Reuses the shape of codex's sandboxing / linux-sandbox (ExecSpec,
ExecOutput) but adds a provider abstraction so the same agent runtime can
run under Docker (default), Kata, or Cube sandboxes.
Each provider is a thin CLI runner: it shells out to the platform binary
(docker, docker --runtime=kata, cube, …). This keeps the crate
self-contained and runnable wherever the corresponding CLI is installed,
while the [Sandbox] trait is the stable seam the rest of the platform
depends on (see docs/adr/0003-sandbox-providers.md).