fix(sandbox): add --map-auto flag for unshare commands

[Einspanner123]

Summary

Fixes sandbox namespace detection and activation on systems where unshare --map-root-user alone fails because the kernel blocks direct uid_map writes by non-root users (observed on util-linux 2.39.3).

Root Cause

unshare --map-root-user tries to write directly to /proc/self/uid_map, which the kernel rejects with EPERM for unprivileged users on some configurations. The --map-auto flag tells unshare to use the newuidmap SUID helper with /etc/subuid delegations instead, which is the recommended approach for unprivileged user namespaces.

On systems where --map-root-user already works natively, the additional --map-auto flag is a safe no-op.

Changes

Two one-line additions in rust/crates/runtime/src/sandbox.rs:

1. Detection probe (line 296): --map-root-user → --map-root-user --map-auto
2. Sandbox launch args (line 225): add --map-auto to the unshare argument list

Verification

• cargo build --release succeeds
• cargo test --package runtime -- sandbox — all 7 tests pass
• claw doctor shows sandbox as supported and active on a system where it previously failed

Before / After

# Before
Sandbox   warn   sandbox was requested but is not currently active
                 fallback: namespace isolation unavailable (requires Linux with `unshare`)

# After
Sandbox   ok     sandbox protections are active
                 supported=true  active=true  filesystem-mode=workspace-only

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