chore: track upstream main branch for vendor submodules

- Add branch = main to each submodule in .gitmodules
- Add GitHub Actions workflow that checks every 6 hours for
  upstream updates and opens a PR automatically

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/update-submodules.yml

@@ -0,0 +1,50 @@
+name: Update vendor submodules
+
+on:
+  schedule:
+    - cron: '0 */6 * * *'  # Every 6 hours
+  workflow_dispatch:        # Manual trigger
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+          fetch-depth: 0
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Update submodules to latest main
+        run: git submodule update --remote --merge
+
+      - name: Check for changes
+        id: check
+        run: |
+          if git diff --quiet; then
+            echo "changed=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "changed=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Create PR with updates
+        if: steps.check.outputs.changed == 'true'
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          BRANCH="chore/update-submodules-$(date +%Y%m%d-%H%M%S)"
+          git checkout -b "$BRANCH"
+          git add vendor/
+          git commit -m "chore: update vendor submodules to latest main"
+          git push origin "$BRANCH"
+          gh pr create \
+            --title "chore: update vendor submodules" \
+            --body "Automated submodule update to latest upstream main." \
+            --base main \
+            --head "$BRANCH"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.gitmodules

@@ -1,9 +1,12 @@
 [submodule "vendor/midstream"]
 	path = vendor/midstream
 	url = https://github.com/ruvnet/midstream
+	branch = main
 [submodule "vendor/ruvector"]
 	path = vendor/ruvector
 	url = https://github.com/ruvnet/ruvector
+	branch = main
 [submodule "vendor/sublinear-time-solver"]
 	path = vendor/sublinear-time-solver
 	url = https://github.com/ruvnet/sublinear-time-solver
+	branch = main