workflows: pr-trigger: add

Add a workflow for triggering workflows named "build-<package>.yml"
(and if present, "test-<package>.yml") with desired release tags for a
given package. For automated upgrade checks and manual changes, this
means that the tags only need to be specified in the PR description when
opened.

This is based on the process in wheel_builder where a child pipeline
would be generated automatically when a merge request was submitted with
one or more package version tags listed as e.g. "Trigger:
polars:py-1.42.1" in the MR description.

AI-Generated: Uses Claude Code Sonnet 4.7

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/pr-trigger.yml

@@ -0,0 +1,72 @@
+# SPDX-FileCopyrightText: 2026 The RISE Project
+# SPDX-License-Identifier: MIT
+
+name: PR package build trigger
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+permissions:
+  contents: read
+  actions: write
+  pull-requests: read
+
+jobs:
+  dispatch-triggers:
+    runs-on: ubuntu-latest
+    if: contains(github.event.pull_request.body, 'Trigger:')
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          package_version: ${{ github.event.pull_request.head.sha }}
+
+      - name: Dispatch build/test workflows for Trigger directives
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_BODY: ${{ github.event.pull_request.body }}
+          HEAD_REF: ${{ github.event.pull_request.head.ref }}
+          HEAD_REPO: ${{ github.event.pull_request.head.repo.full_name }}
+          BASE_REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+
+          if [ "$HEAD_REPO" != "$BASE_REPO" ]; then
+            echo "PR from fork ($HEAD_REPO); cannot dispatch workflows against fork ref. Skipping."
+            exit 0
+          fi
+
+          triggers=$(printf '%s\n' "$PR_BODY" | grep -oE '^Trigger:[[:space:]]*[^[:space:]]+' || true)
+
+          if [ -z "$triggers" ]; then
+            echo "No Trigger: directives found."
+            exit 0
+          fi
+
+          printf '%s\n' "$triggers" | while IFS= read -r line; do
+            directive=${line#Trigger:}
+            directive=${directive# }
+            package_name=${directive%:*}
+            package_version=${directive#*:}
+
+            if [ -z "$package_name" ] || [ -z "$package_version" ] || [ "$package_name" = "$package_version" ]; then
+              echo "Skipping malformed directive: $line"
+              continue
+            fi
+
+            build_wf=".github/workflows/build-${package_name}.yml"
+            test_wf=".github/workflows/test-${package_name}.yml"
+
+            if [ ! -f "$build_wf" ]; then
+              echo "No build workflow for $package_name at $build_wf; skipping."
+              continue
+            fi
+
+            echo "Dispatching build-${package_name}.yml on $HEAD_REF with version=$package_version"
+            gh workflow run "build-${package_name}.yml" --ref "$HEAD_REF" -f "version=$package_version"
+
+            if [ -f "$test_wf" ]; then
+              echo "Dispatching test-${package_name}.yml on $HEAD_REF with version=$package_version"
+              gh workflow run "test-${package_name}.yml" --ref "$HEAD_REF" -f "version=$package_version"
+            fi
+          done