chore: remove Caddy Compatibility Gate workflow

.github/workflows/caddy-compat.yml

@@ -1,58 +0,0 @@
-name: Caddy Compatibility Gate
-
-on:
-  pull_request:
-    paths:
-      - Dockerfile
-      - scripts/caddy-compat-matrix.sh
-      - docs/plans/current_spec.md
-      - .github/workflows/caddy-compat.yml
-  workflow_dispatch:
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: true
-
-permissions:
-  contents: read
-
-jobs:
-  compatibility-matrix:
-    name: PR-1 Compatibility Matrix (Candidate)
-    runs-on: ubuntu-latest
-    timeout-minutes: 90
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
-
-      - name: Set up Go
-        uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6
-        with:
-          go-version: '1.26.0'
-          cache-dependency-path: backend/go.sum
-
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@c7c53464625b32c7a7e944ae62b3e17d2b600130 # v3.7.0
-
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
-
-      - name: Run deterministic compatibility matrix gate
-        run: |
-          bash scripts/caddy-compat-matrix.sh \
-            --candidate-version 2.11.1 \
-            --patch-scenarios A,B,C \
-            --platforms linux/amd64,linux/arm64 \
-            --smoke-set boot_caddy,plugin_modules,config_validate,admin_api_health \
-            --output-dir test-results/caddy-compat \
-            --docs-report docs/reports/caddy-compatibility-matrix.md
-
-      - name: Upload compatibility artifacts
-        if: always()
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
-        with:
-          name: caddy-compatibility-artifacts
-          path: |
-            test-results/caddy-compat/**
-            docs/reports/caddy-compatibility-matrix.md
-          retention-days: 14