fix: firmware CI — source IDF environment and use v5.2 image

The espressif/idf container requires `. $IDF_PATH/export.sh` to put
idf.py on PATH. GitHub Actions container: runs with plain sh which
skips the container entrypoint. Also downgrade from v5.4 to v5.2
which matches our local Docker build environment.

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/firmware-ci.yml

@@ -15,7 +15,7 @@ jobs:
     name: Build ESP32-S3 Firmware
     runs-on: ubuntu-latest
     container:
-      image: espressif/idf:v5.4
+      image: espressif/idf:v5.2
 
     steps:
       - uses: actions/checkout@v4
@@ -23,6 +23,7 @@ jobs:
       - name: Build firmware
         working-directory: firmware/esp32-csi-node
         run: |
+          . $IDF_PATH/export.sh
           idf.py set-target esp32s3
           idf.py build