fix: handle existing sync PRs in upstream-sync workflow

Add idempotency to the merge-upstream job:

1. Check for existing open PR from chore/upstream-sync-* branch
2. If PR exists and branch is up to date with upstream → skip
3. If PR exists but upstream has new commits → checkout existing
   branch, merge new upstream commits, run Claude to review/test
4. If no PR exists → create new branch (existing behavior)

Also update Claude's prompt to handle both create and update PR
cases, so it knows to push to the existing PR branch instead of
creating a duplicate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/workflows/upstream-sync.yml

@@ -187,6 +187,27 @@ jobs:
       with:
         fetch-depth: '0'
         token: ${{ secrets.CLAUDE_BOT_TOKEN }}
+    - name: Check for existing sync PR
+      id: existing-pr
+      run: |-
+        # Search for open PRs from chore/upstream-sync-* branches
+        RESPONSE=$(curl -s \
+          -H "Authorization: token ${{ secrets.CLAUDE_BOT_TOKEN }}" \
+          -H "Content-Type: application/json" \
+          "${{ env.GITHUB_API_URL }}/repos/${{ github.repository }}/pulls?state=open&limit=50")
+
+        PR_BRANCH=$(echo "$RESPONSE" | jq -r '[.[] | select(.head.ref | startswith("chore/upstream-sync-"))] | .[0].head.ref // ""')
+
+        if [ -n "$PR_BRANCH" ]; then
+          PR_NUMBER=$(echo "$RESPONSE" | jq -r '[.[] | select(.head.ref | startswith("chore/upstream-sync-"))] | .[0].number')
+          echo "Found existing sync PR #$PR_NUMBER on branch $PR_BRANCH"
+          echo "pr_exists=true" >> "$GITHUB_OUTPUT"
+          echo "pr_branch=$PR_BRANCH" >> "$GITHUB_OUTPUT"
+          echo "pr_number=$PR_NUMBER" >> "$GITHUB_OUTPUT"
+        else
+          echo "No existing sync PR found"
+          echo "pr_exists=false" >> "$GITHUB_OUTPUT"
+        fi
     - name: Prepare upstream merge
       id: prepare
       run: |-
@@ -196,6 +217,29 @@ jobs:
         git remote add upstream https://github.com/anthropics/claude-code-action.git
         git fetch upstream main --quiet
 
+        # Check if existing PR branch is already up to date with upstream
+        if [ "${{ steps.existing-pr.outputs.pr_exists }}" = "true" ]; then
+          PR_BRANCH="${{ steps.existing-pr.outputs.pr_branch }}"
+          git checkout "$PR_BRANCH"
+
+          # Check if upstream/main is already merged into the PR branch
+          BEHIND=$(git rev-list "$PR_BRANCH"..upstream/main --count)
+          if [ "$BEHIND" -eq 0 ]; then
+            echo "Existing PR branch $PR_BRANCH is already up to date with upstream"
+            echo "skip=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          echo "Existing PR branch $PR_BRANCH is $BEHIND commits behind upstream — updating"
+          BRANCH="$PR_BRANCH"
+        else
+          BRANCH="chore/upstream-sync-$(date +%Y%m%d)"
+          git checkout -b "$BRANCH"
+        fi
+
+        echo "skip=false" >> "$GITHUB_OUTPUT"
+        echo "branch=$BRANCH" >> "$GITHUB_OUTPUT"
+
         # Files our fork has modified vs upstream
         FORK_FILES=$(git log upstream/main..main --name-only --pretty=format: | sort -u | grep -v '^$')
         echo "=== Fork-modified files ==="
@@ -226,10 +270,6 @@ jobs:
         echo "${AFFECTED:-(none)}" >> "$GITHUB_OUTPUT"
         echo "EOF" >> "$GITHUB_OUTPUT"
 
-        BRANCH="chore/upstream-sync-$(date +%Y%m%d)"
-        git checkout -b "$BRANCH"
-        echo "branch=$BRANCH" >> "$GITHUB_OUTPUT"
-
         if git merge upstream/main --no-edit; then
           echo "merge_clean=true" >> "$GITHUB_OUTPUT"
           echo "Merge completed cleanly"
@@ -252,9 +292,12 @@ jobs:
           curl -fsSL https://bun.sh/install | bash -s "bun-v${BUN_VERSION}"
           echo "$HOME/.bun/bin" >> "$GITHUB_PATH"
         fi
+      if: steps.prepare.outputs.skip != 'true'
     - name: Install dependencies
       run: bun install
+      if: steps.prepare.outputs.skip != 'true'
     - name: 'Claude: review merge and create PR'
+      if: steps.prepare.outputs.skip != 'true'
       uses: https://${{ secrets.READ_REPO_TOKEN }}@git.wxs.ro/actions/claude-code-action@main
       with:
         prompt: |-
@@ -264,6 +307,7 @@ jobs:
           - Branch: ${{ steps.prepare.outputs.branch }}
           - ${{ needs.check-upstream.outputs.behind }} commits behind upstream, ${{ needs.check-upstream.outputs.ahead }} commits ahead (our Forgejo changes)
           - Tracking issue: #${{ needs.check-upstream.outputs.issue_number }}
+          - Existing PR: ${{ steps.existing-pr.outputs.pr_exists }} (if true, PR #${{ steps.existing-pr.outputs.pr_number }} already exists — update it instead of creating a new one)
           - Merge was attempted. Status: ${{ steps.prepare.outputs.merge_clean }} (true = clean, false = has conflicts)
 
           ## Impact analysis
@@ -294,8 +338,10 @@ jobs:
           2. Run typecheck: bun run typecheck
           3. Check formatting: bun run format:check — if issues, fix with bun run format
           4. Commit any remaining changes and push the branch
-          5. Create a PR targeting main via the Forgejo API:
-             POST $GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls
+          5. Push the branch, then create or update the PR:
+             - If existing PR is "true": PR #${{ steps.existing-pr.outputs.pr_number }} already exists. Just push the branch — the PR updates automatically.
+               Optionally update the PR body via PATCH $GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls/${{ steps.existing-pr.outputs.pr_number }}
+             - If existing PR is "false": Create a new PR via POST $GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls
              Auth: token $GITHUB_TOKEN
              Title: "chore: merge ${{ needs.check-upstream.outputs.behind }} upstream commits from anthropics/claude-code-action"
              Body should include: