feat(bottle): opt-in gVisor runtime per bottle

Bottles can now set "runtime": "runsc" to launch the agent container
under gVisor instead of runc, adding a userspace syscall barrier
between the agent and the host kernel. Default is runc (Docker
default). Pipelock stays on the default runtime per the research doc's
minimum-diff prescription.

The launcher verifies runsc is registered with the daemon before
launch, surfaces the runtime in the preflight plan, and dies with an
install pointer (and a macOS-not-supported note) when runsc is
requested but unavailable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/docker.py

@@ -19,6 +19,22 @@ def require_docker() -> None:
         die("docker not found")
 
 
+def require_runsc() -> None:
+    """Fail with an install pointer if the `runsc` (gVisor) runtime is
+    not registered with the local Docker daemon. Called when a bottle
+    sets `runtime: "runsc"`."""
+    result = subprocess.run(
+        ["docker", "info", "--format", "{{json .Runtimes}}"],
+        capture_output=True,
+        text=True,
+    )
+    if result.returncode != 0 or "runsc" not in result.stdout:
+        info("This bottle requested runtime 'runsc' but the gVisor runtime is not registered with Docker.")
+        info("Install gVisor and register it with the daemon:  https://gvisor.dev/docs/user_guide/install/")
+        info("On macOS, gVisor is not available natively; remove 'runtime' from the bottle or run on Linux.")
+        die("runsc runtime not available")
+
+
 def image_exists(ref: str) -> bool:
     return _silent_run(["docker", "image", "inspect", ref]) == 0