feat(ci): add regen-stubs workflow for decree releases

Adds a repository_dispatch receiver that auto-regens proto stubs when
a new decree version is released and opens a PR with the updated stubs.

Part of opendecree/decree#157.

Author: zeevdr

.github/workflows/regen-stubs.yml

@@ -0,0 +1,40 @@
+name: Regen stubs
+
+on:
+  repository_dispatch:
+    types: [decree-released]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  regen-stubs:
+    name: Regen stubs for ${{ github.event.client_payload.version }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          persist-credentials: true
+
+      - name: Regen proto stubs
+        run: make generate
+        # Python stubs are generated from buf; make generate runs buf generate
+
+      - name: Run tests
+        run: make test
+
+      - name: Open PR
+        # peter-evans/create-pull-request@v7
+        uses: peter-evans/create-pull-request@aa523f9db61947bd5e06efd77870ef065eae32cc
+        with:
+          token: ${{ secrets.PROJECT_TOKEN }}
+          commit-message: "chore: regen stubs for decree ${{ github.event.client_payload.version }}"
+          title: "chore: Regen stubs for decree ${{ github.event.client_payload.version }}"
+          body: |
+            Auto-generated by the decree release workflow.
+
+            Triggered by decree release ${{ github.event.client_payload.version }}.
+          labels: dependencies,ci
+          branch: "regen-stubs/${{ github.event.client_payload.version }}"
+          base: main