feat(smolmachines): run backend on Linux

Port the smolmachines backend so BOT_BOTTLE_BACKEND=smolmachines
works on Linux (KVM), not just macOS:

- Preflight gates /dev/kvm presence + accessibility on Linux with
  actionable remediation (kvm module, kvm group).
- smolvm state-DB path is platform-derived (XDG on Linux).
- force_allowlist runs on both platforms and is fail-closed: it
  verifies the persisted TSI allowlist and dies rather than booting
  a VM whose egress confinement it can't confirm. Previously it
  no-oped on Linux, failing OPEN.
- allocate() does per-bottle 127.0.0.<N> scoping on Linux too (no
  ifconfig needed — all of 127/8 is already loopback); only
  ensure_pool's lo0 aliasing stays macOS-only.
- README documents Linux + NixOS host setup.

Linux/KVM integration (the sandbox-escape acceptance gate) is
pending verification on a NixOS host; unit tests cover the new
platform branches.

Issue: #283

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

bot_bottle/backend/smolmachines/util.py

@@ -5,26 +5,58 @@ unit-tested without importing the docker subprocess paths."""
 from __future__ import annotations
 
 import hashlib
+import os
+import platform
 import shutil
 
 from ...log import die
 
+# libkrun's Linux backend drives the guest through KVM, so the host
+# must expose `/dev/kvm` and the invoking user must be able to open
+# it. macOS uses Hypervisor.framework and needs no device node.
+_KVM_DEVICE = "/dev/kvm"
+
 
 def smolmachines_preflight() -> None:
-    """Ensure `smolvm` is on PATH before the launch flow runs.
-    Called from `_resolve_plan`; gives the operator a clear
-    install pointer rather than a cryptic FileNotFoundError
-    later. `gvproxy` is no longer required — see the PRD's design
-    pivot section."""
-    if shutil.which("smolvm") is not None:
-        return
-    die(
-        "BOT_BOTTLE_BACKEND=smolmachines requires `smolvm` on "
-        "PATH. Install with: "
-        "curl -sSL https://smolmachines.com/install.sh | sh. "
-        "To use the legacy Docker backend instead, set "
-        "BOT_BOTTLE_BACKEND=docker or pass --backend=docker."
-    )
+    """Ensure the host can run the smolmachines backend before the
+    launch flow starts. Called from `_resolve_plan`; surfaces a
+    clear, actionable error instead of a cryptic `smolvm` failure
+    deep in launch.
+
+    Checks `smolvm` is on PATH (both platforms) and, on Linux,
+    that `/dev/kvm` exists and is accessible. `gvproxy` is no
+    longer required — see the PRD's design pivot section."""
+    if shutil.which("smolvm") is None:
+        die(
+            "BOT_BOTTLE_BACKEND=smolmachines requires `smolvm` on "
+            "PATH. Install with: "
+            "curl -sSL https://smolmachines.com/install.sh | sh. "
+            "To use the legacy Docker backend instead, set "
+            "BOT_BOTTLE_BACKEND=docker or pass --backend=docker."
+        )
+    if platform.system() == "Linux":
+        _preflight_kvm()
+
+
+def _preflight_kvm() -> None:
+    """Linux-only: libkrun needs `/dev/kvm`. Distinguish 'KVM not
+    enabled' from 'no permission' so the operator knows which to
+    fix."""
+    if not os.path.exists(_KVM_DEVICE):
+        die(
+            f"BOT_BOTTLE_BACKEND=smolmachines needs {_KVM_DEVICE} on "
+            "Linux but it is missing. Enable KVM: load the kvm-intel "
+            "or kvm-amd kernel module (and confirm virtualization is "
+            "enabled in BIOS/firmware). To use the legacy Docker "
+            "backend instead, set BOT_BOTTLE_BACKEND=docker."
+        )
+    if not os.access(_KVM_DEVICE, os.R_OK | os.W_OK):
+        die(
+            f"{_KVM_DEVICE} exists but is not readable/writable by the "
+            "current user. Add your user to the `kvm` group "
+            "(`sudo usermod -aG kvm \"$USER\"`) and re-login, or run "
+            "with access to the device."
+        )
 
 
 def smolmachines_bundle_subnet(slug: str) -> tuple[str, str, str]: