Configure a GitHub Actions matrix build that generates its matrix dynamically from a JSON file in the repository and fans out to per-environment deploy jobs using environment protection rules

Steps

1. Add a setup job that runs a script to read a matrix definition JSON file from the repo and emits it as a job output using echo "matrix=$(cat matrix.json)" >> $GITHUB_OUTPUT
2. Reference the output in the downstream job's strategy.matrix block using fromJson(needs.setup.outputs.matrix) so the fan-out is driven entirely by the file
3. Define each matrix entry with an environment field that matches a GitHub environment name; set environment: ${{ matrix.environment }} on the deploy job so protection rules and required reviewers apply per target
4. Use concurrency: group: deploy-${{ matrix.environment }} with cancel-in-progress: false to prevent concurrent deploys to the same environment while allowing parallel deploys to different environments
5. Add a matrix.include entry for a canary environment with an extra weight field and conditionally adjust the deploy script behavior based on that field using if: matrix.weight
6. Wire a summary job using needs: [deploy] with strategy: fail-fast: false so it runs even when individual matrix legs fail and posts a consolidated status to a Slack webhook