fix: use sed instead of grep -P for BusyBox compatibility in CI

The Alpine-based docker builder uses BusyBox grep which doesn't
support Perl regex (-P). Switch to sed for extracting the agent
version from Maven metadata XML.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -112,9 +112,11 @@ jobs:
       - name: Build and push runtime base image
         run: |
           AGENT_VERSION=$(curl -sf "https://gitea.siegeln.net/api/packages/cameleer/maven/com/cameleer3/cameleer3-agent/1.0-SNAPSHOT/maven-metadata.xml" \
-            | grep -oP '(?<=<value>)[^<]+' | tail -1)
+            | sed -n 's/.*<value>\([^<]*\)<\/value>.*/\1/p' | tail -1)
+          echo "Agent version: $AGENT_VERSION"
           curl -sf -o docker/runtime-base/agent.jar \
             "https://gitea.siegeln.net/api/packages/cameleer/maven/com/cameleer3/cameleer3-agent/1.0-SNAPSHOT/cameleer3-agent-${AGENT_VERSION}-shaded.jar"
+          ls -la docker/runtime-base/agent.jar
           TAGS="-t gitea.siegeln.net/cameleer/cameleer-runtime-base:${{ github.sha }}"
           for TAG in $IMAGE_TAGS; do
             TAGS="$TAGS -t gitea.siegeln.net/cameleer/cameleer-runtime-base:$TAG"