ci: add weekly upstream digest workflow (#37)

Problem: new upstream issues and PRs slip through because there's no
mechanism to surface them — manual polling of stevearc/oil.nvim is
required and easy to forget.

Solution: add a Monday 9am UTC scheduled workflow that reads the
highest stevearc/oil.nvim number from doc/upstream.md, fetches merged
PRs and new open issues/PRs above that threshold via the gh CLI, and
creates a structured digest issue in barrettruth/canola.nvim. No issue
is created when there's nothing new. Falls back to a 30-day window if
doc/upstream.md can't be parsed.

.github/workflows/upstream-digest.yml

@@ -0,0 +1,21 @@
+name: upstream digest
+
+on:
+  schedule:
+    - cron: "0 9 * * 1"
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  contents: read
+
+jobs:
+  digest:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Create upstream digest issue
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: python3 .github/scripts/upstream_digest.py