Add sandbox-wide guest rlimits

[enricoschaaf]

Summary

This adds sandbox-wide guest rlimits that are applied by agentd PID 1 during guest startup.

Today microsandbox only supports per-exec rlimits. That is too late for bootstrap scripts and long-lived daemons started during guest init, because they inherit PID 1's baseline limits before any per-exec override runs.

What changed

• adds sandbox-wide SandboxConfig::rlimits
• adds SandboxBuilder::rlimit(...) and SandboxBuilder::rlimit_range(...)
• passes sandbox-wide rlimits into the guest via MSB_RLIMITS
• applies those limits in agentd before the rest of init runs
• keeps per-exec rlimits available so one-off execs can still override the baseline

Why this shape

This fixes the real problem for bootstrap daemons and long-lived guest services instead of requiring every process to opt into a higher limit individually.

Validation

• cargo test --manifest-path /tmp/msb-baseline/crates/agentd/Cargo.toml --lib
• cargo test --manifest-path /tmp/msb-baseline/crates/microsandbox/Cargo.toml --lib