Replace obsolete MQTT broker image with custom build, add connection retry and upgrade docs

- Add custom Dockerfile for meshcore-mqtt-broker (Node 22 Alpine, built from michaelhart/meshcore-mqtt-broker source)
- Add GitHub Actions workflow for weekly multi-arch MQTT broker image builds
- Add local build script (etc/docker/meshcore-mqtt-broker/build.sh)
- Update docker-compose.yml and test compose to use new ghcr.io image
- Add MQTT connection retry logic with exponential backoff to collector subscriber
- Create UPGRADING.md with migration guide for breaking changes
- Update README.md and AGENTS.md for accuracy (Python 3.14, removed commands, fixed MQTT defaults)
- Remove obsolete files (etc/mosquitto.conf, .agentmap.yaml, CLAUDE.md)

.github/workflows/docker-mqtt-broker.yml

@@ -0,0 +1,75 @@
+name: Build MQTT Broker Image
+
+on:
+  schedule:
+    - cron: "0 4 * * 0"
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: "Upstream ref to build (branch, tag, or SHA)"
+        required: false
+        default: "main"
+
+env:
+  REGISTRY: ghcr.io
+  UPSTREAM_REPO: michaelhart/meshcore-mqtt-broker
+  IMAGE_NAME: ipnet-mesh/meshcore-mqtt-broker
+
+jobs:
+  build:
+    name: Build and Push MQTT Broker
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout this repo (sparse)
+        uses: actions/checkout@v6
+        with:
+          sparse-checkout: |
+            etc/docker/meshcore-mqtt-broker
+
+      - name: Checkout upstream source
+        uses: actions/checkout@v6
+        with:
+          repository: ${{ env.UPSTREAM_REPO }}
+          ref: ${{ inputs.ref || 'main' }}
+          path: upstream
+
+      - name: Copy Dockerfile into upstream source
+        run: cp etc/docker/meshcore-mqtt-broker/Dockerfile upstream/Dockerfile
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v4
+
+      - name: Log in to Container Registry
+        uses: docker/login-action@v4
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v6
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v7
+        with:
+          context: ./upstream
+          file: ./upstream/Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max