microsandbox: Secret values exposed in world-readable process arguments

microsandbox is an easy, fast, local-first microVM runtime and library. Prior to 0.5.10, sdk/rust/lib/runtime/spawn.rs serializes NetworkConfig secret values into the --network-config argument and passes per-sandbox secrets through repeated --env arguments accepted by crates/cli/lib/sandbox_cmd.rs. Other local users or co-resident processes can read these values through the host process table, including /proc process command lines on Linux and process listings on Linux and macOS, for the lifetime of the sandbox. Exploitation does not require code execution inside the sandbox or access to the spawning user's session, and can disclose host-side API keys, tokens, and environment secrets on shared hosts, CI runners, and developer systems. This issue is fixed in version 0.5.10.