Add distro/ubuntuCore for UC26 snap and image builds.

Centralize salmanoff snapcraft, dangerous-model image scripts, and QEMU
workflow so UC26 can be reproduced from the SMO repo without ubuntu-core-practice.

Co-authored-by: Cursor <cursoragent@cursor.com>

distro/ubuntuCore/scripts/fetch-model.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# Refresh the signed reference model from snapcore/models on GitHub.
+set -euo pipefail
+
+UC_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+MODEL="${UC_ROOT}/models/ubuntu-core-26-amd64.model"
+URL="https://raw.githubusercontent.com/snapcore/models/master/ubuntu-core-26-amd64.model"
+
+mkdir -p "$(dirname "$MODEL")"
+curl -fsSL -o "$MODEL" "$URL"
+echo "Updated: $MODEL"
+head -15 "$MODEL"