fix(engine/buildx): recipe used reserved 'default' builder name

'docker buildx create --name default' errors with 'default is a
reserved name and cannot be used to identify builder instance'.
The bundled builder always exists under that name; the recipe
should switch to it (buildx use default), not try to recreate it.

For the count==0 driver-rebuild branch, the new builder needs a
non-reserved name — using 'decnet-builder' as the example.

decnet/engine/deployer.py

@@ -203,17 +203,17 @@ def _buildx_recovery_hint(*, leaked_mounts: int, original_stderr: str = "") -> s
             "  3. for m in $(mount | awk '$3 ~ /buildkit-mount/ {print $3}'); do sudo umount -l \"$m\"; done\n"
             "  4. rm -rf ~/.docker/buildx/activity\n"
             "  5. sudo systemctl start docker\n"
-            "  6. docker buildx create --use --name default && docker buildx inspect --bootstrap"
+            "  6. docker buildx use default   # bundled builder is reserved-named; switch to it"
         )
     else:
         fix = (
             "No leaked mounts (count=0) — the buildx driver state "
             "itself is inconsistent.\n"
             "Recovery:\n"
-            "  1. docker buildx rm default 2>/dev/null\n"
-            "  2. rm -rf ~/.docker/buildx/activity ~/.docker/buildx/instances/default\n"
-            "  3. docker buildx create --use --name default\n"
-            "  4. docker buildx inspect --bootstrap"
+            "  1. rm -rf ~/.docker/buildx/activity ~/.docker/buildx/instances/*\n"
+            "  2. docker buildx create --name decnet-builder --use --bootstrap\n"
+            "     (the name 'default' is reserved by Docker — pick anything else)\n"
+            "  3. docker buildx inspect"
         )
     tail = "See wiki: Troubleshooting → 'Buildx leaked mounts'."
     parts = [head, fix, tail]

tests/fleet/test_deployer.py

@@ -176,7 +176,7 @@ class TestComposeWithRetry:
         )
         with pytest.raises(subprocess.CalledProcessError) as ei:
             deployer._compose_with_retry("up", "--build")
-        assert "buildx create --use" in ei.value.stderr
+        assert "buildx create --name decnet-builder" in ei.value.stderr
         assert "umount" not in ei.value.stderr
         assert "No leaked mounts (count=0)" in ei.value.stderr