feat: add NixOS support

[Filirom1]

Problem

On NixOS, executables live under /nix/store and PATH entries such as /run/current-system/sw/bin are symlink chains into the store. Inside the bwrap sandbox, /run is replaced with a tmpfs, so those symlinks become dangling — tools cannot be found and the sandbox fails.

Additionally, the network namespace probe hardcoded /bin/true which does not exist on NixOS, causing greywall check to falsely report ✗ network isolation.

Fixes #81. Related to #22.

Changes

• linux_features.go: resolve true from PATH instead of hardcoding /bin/true so the network namespace probe works on NixOS and any distro without /bin/true.
• linux.go: mount /nix read-only (like /usr, /opt); resolve shell and sleep paths through resolveToolPath so the directory component points into /nix/store rather than the hidden /run/current-system/sw/bin.
• linux_landlock.go: add /nix to Landlock read-allowed paths.
• sanitize.go: rewrite PATH entries through symlinks before passing them into the sandbox, so the sandbox PATH is consistent with what is actually mounted.

Tests

On NixOS:

$ cat /etc/os-release |grep PRETTY
PRETTY_NAME="NixOS 25.11 (Xantusia)"

Before

$ greywall --linux-features
Linux Sandbox Features:
  Kernel: 6.6
  Bubblewrap (bwrap): true
  Socat: true
  Network namespace (--unshare-net): false
  Seccomp: true (log level: 2)
  Landlock: true (ABI v3)
  eBPF: false (CAP_BPF: false, root: false)
  ip (iproute2): true
  /dev/net/tun: true
  tun2socks: true (embedded)

Feature Status:
  ✓ Minimum requirements met (bwrap + socat)
  ⚠ Network namespace unavailable (containerized environment?)
    Sandbox will still work but with reduced network isolation.
    This is common in Docker, GitHub Actions, and other CI systems.
  ○ Transparent proxy not available (needs ip, /dev/net/tun, network namespace)
  ✓ Landlock available for enhanced filesystem control
  ✓ Violation monitoring available
  ○ eBPF monitoring not available (needs CAP_BPF or root)
  
$  greywall -- claude -p '1+1'
bwrap: execvp /nix/store/lfbzxs5wyqd2122mpbj5azkxhxspw9cd-bash-interactive-5.3p3/bin/bash: No such file or directory

After

$ greywall --linux-features
Linux Sandbox Features:
  Kernel: 6.6
  Bubblewrap (bwrap): true
  Socat: true
  Network namespace (--unshare-net): true
  Seccomp: true (log level: 2)
  Landlock: true (ABI v3)
  eBPF: false (CAP_BPF: false, root: false)
  ip (iproute2): true
  /dev/net/tun: true
  tun2socks: true (embedded)

Feature Status:
  ✓ Minimum requirements met (bwrap + socat)
  ✓ Network namespace isolation available
  ✓ Transparent proxy available (tun2socks + TUN device)
  ✓ Landlock available for enhanced filesystem control
  ✓ Violation monitoring available
  ○ eBPF monitoring not available (needs CAP_BPF or root)
  
  $ greywall -- claude -p '1+1'
2