fix: build engram from source in Docker image

The engram binary was never published to Artifact Registry (foundation-dev/engram
package does not exist). Updated Dockerfile to build engram from source using the
El SDK packages (el-elc, el-runtime-c, el-runtime-h) from foundation-dev.

Also:
- Switch runtime base to Ubuntu 24.04 (GLIBC 2.39 required by elc-compiled binaries)
- Add -lm to engram link flags (el_runtime.c uses pow/sqrt/log/sin/cos/exp)
- Update deploy-gke.yaml to clone neuron-technologies/engram into build context

.gitea/workflows/deploy-gke.yaml

@@ -87,6 +87,15 @@ jobs:
           echo "slot=${SLOT}" >> "$GITEA_OUTPUT"
           echo "  Deploying to slot: ${SLOT}"
 
+      - name: Clone engram source for Docker build context
+        run: |
+          # The Dockerfile builds engram from source (no published AR package).
+          # Clone the engram repo into ./engram/ so it's available in the build context.
+          git clone http://34.31.145.131/neuron-technologies/engram.git \
+            --depth=1 --branch=main \
+            engram
+          echo "Engram source ready at ./engram/src/server.el"
+
       - name: Build and push Docker image
         env:
           GCP_SA_KEY: ${{ secrets.GCP_SA_KEY }}