Refactor CI into composite actions + repo variables

The workflow was mostly inline bash. Moves the logic into reusable
composite actions under .gitea/actions/, keeps host paths in the
YOCTO_HOME repo variable, adds a workflow_dispatch skip-build option
and publishes step outputs (build dir, image list, kas path).

.gitea/actions/check-build-host/action.yml

@@ -0,0 +1,37 @@
+name: check-build-host
+description: >-
+  Report the build host and fail fast when executables, CPU or disk that the
+  build needs are missing - a mistake should cost seconds, not hours.
+inputs:
+  tools:
+    description: Space separated executables that must be on PATH
+    required: false
+    default: git python3
+  min-disk-gb:
+    description: Warn when the build filesystem has less than this much free
+    required: false
+    default: "100"
+runs:
+  using: composite
+  steps:
+    - name: Host report
+      shell: bash
+      run: |
+        set -euo pipefail
+        echo "os     : $(. /etc/os-release && echo "$PRETTY_NAME") ($(uname -m), $(uname -r))"
+        echo "user   : $(id -un)  home=$HOME"
+        echo "cpu    : $(nproc) cores / $(awk '/MemTotal/{printf "%.0fG", $2/1048576}' /proc/meminfo) ram"
+        df -h . | tail -1 | sed 's/^/disk   : /'
+        missing=""
+        for tool in ${{ inputs.tools }}; do
+          command -v "$tool" >/dev/null 2>&1 || missing="$missing $tool"
+        done
+        if [ -n "$missing" ]; then
+          echo "::error::Host is missing:$missing"
+          echo "Install them on the runner host, e.g. sudo apt-get install -y $missing"
+          exit 1
+        fi
+        free_gb="$(df --output=avail -BG . | tail -1 | tr -d ' ')"
+        if [ "$free_gb" -lt "${{ inputs.min-disk-gb }}" ]; then
+          echo "::warning::only ${free_gb}G free on $(df --output=target . | tail -1); expected >= ${{ inputs.min-disk-gb }}G"
+        fi