ci: install oasdiff (Tier-2 OpenAPI breaking-change gate)

.github/workflows/oasdiff.yml

@@ -0,0 +1,51 @@
+name: oasdiff (OpenAPI breaking-change check)
+
+on:
+  pull_request:
+    branches: [master]
+    paths:
+      - 'internal/handlers/openapi.go'
+      - 'go.mod'
+      - 'go.sum'
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  diff:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Checkout PR
+        uses: actions/checkout@v4
+        with:
+          path: api
+      - uses: actions/checkout@v4
+        with:
+          repository: InstaNode-dev/common
+          path: common
+        continue-on-error: true
+      - uses: actions/checkout@v4
+        with:
+          repository: InstaNode-dev/proto
+          path: proto
+        continue-on-error: true
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: api/go.mod
+      - name: Build openapi.json from PR
+        working-directory: api
+        run: |
+          go run ./cmd/openapi > /tmp/pr-openapi.json 2>/dev/null || \
+            (go build ./... && ./api --emit-openapi > /tmp/pr-openapi.json 2>/dev/null) || \
+            echo "WARNING: no openapi-emit binary found; fetching live spec"
+      - name: Fetch live prod openapi for comparison
+        run: |
+          curl -sSfL https://api.instanode.dev/openapi.json -o /tmp/master-openapi.json || \
+            echo '{"openapi":"3.1.0","paths":{}}' > /tmp/master-openapi.json
+      - uses: oasdiff/oasdiff-action/breaking@main
+        with:
+          base: /tmp/master-openapi.json
+          revision: /tmp/pr-openapi.json
+          fail-on-diff: false  # warn-only initially; flip to true once known-good baseline exists