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/setup-kas/action.yml

@@ -0,0 +1,34 @@
+name: setup-kas
+description: Create or reuse the persistent python venv that holds kas.
+inputs:
+  requirement:
+    description: pip requirement for kas (pinned, e.g. kas==5.5)
+    required: false
+    default: kas
+outputs:
+  kas:
+    description: Absolute path of the kas executable
+    value: ${{ steps.venv.outputs.kas }}
+  reused:
+    description: "true when the venv already existed (warm run)"
+    value: ${{ steps.venv.outputs.reused }}
+runs:
+  using: composite
+  steps:
+    - name: Prepare venv
+      id: venv
+      shell: bash
+      run: |
+        set -euo pipefail
+        BUILD_ROOT="${YOCTO_HOME:-$HOME/yocto-build}"
+        VENV="$BUILD_ROOT/venv"
+        mkdir -p "$BUILD_ROOT"
+        if [ -d "$VENV" ]; then
+          echo "reused=true" >> "$GITHUB_OUTPUT"
+        else
+          echo "reused=false" >> "$GITHUB_OUTPUT"
+          python3 -m venv "$VENV"
+        fi
+        "$VENV/bin/pip" install --quiet --upgrade "${{ inputs.requirement }}"
+        echo "kas=$VENV/bin/kas" >> "$GITHUB_OUTPUT"
+        echo "kas $($VENV/bin/kas --version) in $VENV"