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/build-image/action.yml

@@ -0,0 +1,117 @@
+name: build-image
+description: >-
+  Build a kas project in a persistent directory and expose the flashable
+  images. The workspace is synced into the build dir instead of built in
+  place, because checkout cleans ignored files (build/, extern/) and bitbake
+  reuse depends on sstate-cache surviving between runs.
+inputs:
+  project:
+    description: kas configuration file, relative to the repository
+    required: false
+    default: .config.yaml
+  machine:
+    description: Machine the images are deployed for
+    required: false
+    default: raspberrypi4-64
+  image-prefix:
+    description: Artifact filename prefix in the deploy dir
+    required: false
+    default: rpilinux-image
+  skip-build:
+    description: Set to true to only collect images from an existing build dir
+    required: false
+    default: "false"
+outputs:
+  build-dir:
+    description: Persistent project directory on the runner
+    value: ${{ steps.locate.outputs.build-dir }}
+  image-dir:
+    description: Deploy dir bitbake wrote to
+    value: ${{ steps.locate.outputs.image-dir }}
+  images:
+    description: Newline separated artifact filenames
+    value: ${{ steps.collect.outputs.images }}
+runs:
+  using: composite
+  steps:
+    - name: Sync workspace into persistent build dir
+      if: ${{ inputs.skip-build != 'true' }}
+      shell: bash
+      run: |
+        set -euo pipefail
+        SRC="$(pwd)"
+        DST="${YOCTO_HOME:-$HOME/yocto-build}/$(basename "$GITHUB_REPOSITORY")"
+        # checkout deletes ignored files, so the build tree must live elsewhere
+        if [ "$SRC" = "$DST" ]; then
+          echo "::error::workspace == build dir ($SRC); point YOCTO_HOME somewhere else"
+          exit 1
+        fi
+        echo "syncing $SRC -> $DST"
+        mkdir -p "$DST"
+        # .git stays (kas requires a repo); extern/ + build/ only exist in DST,
+        # so --delete cannot reach them
+        rsync -a --delete --exclude /build/ --exclude /extern/ ./ "$DST/"
+
+    - name: kas build
+      if: ${{ inputs.skip-build != 'true' }}
+      shell: bash
+      run: |
+        set -euo pipefail
+        cd "${YOCTO_HOME:-$HOME/yocto-build}/$(basename "$GITHUB_REPOSITORY")"
+        git config --global --add safe.directory "$PWD" || true
+        "${YOCTO_HOME:-$HOME/yocto-build}/venv/bin/kas" build
+
+    - name: Locate deploy dir
+      id: locate
+      shell: bash
+      run: |
+        set -euo pipefail
+        BUILD_DIR="${YOCTO_HOME:-$HOME/yocto-build}/$(basename "$GITHUB_REPOSITORY")"
+        IMAGE_DIR="$(find "$BUILD_DIR/build" -type d -path "*/tmp/deploy/images/${{ inputs.machine }}" | head -1)"
+        if [ -z "$IMAGE_DIR" ]; then
+          echo "::error::no deploy dir for ${{ inputs.machine }} under $BUILD_DIR/build"
+          exit 1
+        fi
+        echo "build-dir=$BUILD_DIR" >> "$GITHUB_OUTPUT"
+        echo "image-dir=$IMAGE_DIR" >> "$GITHUB_OUTPUT"
+
+    - name: Collect artifacts
+      id: collect
+      shell: bash
+      run: |
+        set -euo pipefail
+        IMAGE_DIR="${{ steps.locate.outputs.image-dir }}"
+        shopt -s nullglob
+        # only the timestamped names: the short ones are hardlinks to the same
+        # inode and would upload every image twice
+        FILES=("${IMAGE_DIR}/${{ inputs.image-prefix }}."*.rootfs-[0-9]*.wic.* \
+               "${IMAGE_DIR}/${{ inputs.image-prefix }}-*.rootfs-[0-9]*.wic.*")
+        if [ "${#FILES[@]}" -eq 0 ]; then
+          echo "::error::no flashable .wic image for ${{ inputs.machine }} in $IMAGE_DIR"
+          exit 1
+        fi
+        mkdir -p dist
+        cp -v "${FILES[@]}" dist/
+        ( cd dist && sha256sum * > SHA256SUMS )
+        ls -lh dist
+        printf 'images=%s\n' "$(ls dist | tr '\n' ' ')" >> "$GITHUB_OUTPUT"
+
+    - name: Summarise
+      shell: bash
+      run: |
+        set -euo pipefail
+        SUMMARY="${GITHUB_STEP_SUMMARY:-}"
+        [ -n "$SUMMARY" ] || exit 0
+        {
+          echo "## ${{ inputs.image-prefix }} for ${{ inputs.machine }}"
+          echo
+          echo "| artifact | sha256 |"
+          echo "| --- | --- |"
+          ( cd dist && while read -r sum file; do
+              [ "$file" = SHA256SUMS ] || echo "| \`$file\` | \`${sum:0:16}\` |"
+            done < SHA256SUMS )
+          echo
+          echo "Flash with \`bzcat <image> | sudo dd of=/dev/sdX bs=4M status=progress\`."
+          echo
+          echo "Incremental build tree: \`${{ steps.locate.outputs.build-dir }}\`"
+        } >> "$SUMMARY"