feat(sandbox): start mounted sandboxes lazily and stop them when idle

Problem

Configuring sandbox: { provider } starts the external sandbox as soon as the AgentOS VM boots, even when the agent never reads the mount or calls a sandbox binding.

Desired behavior

• Do not call provider.start() during VM creation.
• Start the provider on the first operation against the sandbox filesystem mount or any agentos-sandbox binding.
• Share one single-flight initializer between filesystem and binding calls so concurrent first use creates exactly one sandbox.
• Add a configurable inactivity timeout that shuts down or suspends the sandbox after no filesystem, binding, or in-flight process-control activity.
• Transparently restart or resume it on the next operation.
• Always shut it down when the AgentOS VM stops, regardless of the inactivity timeout.
• Clean up partially started resources when initialization fails.

Define and document persistence semantics for providers that can suspend/resume versus providers that must destroy/recreate the sandbox. Add native integration coverage for first-use startup, concurrent first use, idle shutdown, restart, and VM disposal.

Trace VM creation, the sandbox filesystem mount, agentos-sandbox bindings, provider.start(), and VM shutdown to map the lifecycle entry points. Define the shared first-use initialization, inactivity handling, persistence semantics, failure cleanup, and disposal behavior, then add native integration coverage for startup, concurrency, idle shutdown, restart, and VM disposal.