ci(openspec): Skip the archive check on non-tip stacked PRs

.github/workflows/pr-check-openspec.yml

@@ -1,14 +1,52 @@
 name: PR OpenSpec Archive Check
 
+# A change is archived exactly once, when ALL of its work has landed. In a
+# stack of PRs, only the tip carries the archived change; the PRs below it still
+# carry the in-flight change directory by design. So this check runs only on the
+# tip of a stack (or a standalone PR) and is skipped on PRs that still have work
+# stacked on top of them. Tip = no other OPEN PR targets this PR's head branch
+# as its base.
 on:
   pull_request:
-    branches: [main]
 
 permissions:
   contents: read
+  pull-requests: read
 
 jobs:
+  stack-position:
+    name: Detect stack position
+    runs-on: ubuntu-latest
+    outputs:
+      is_tip: ${{ steps.detect.outputs.is_tip }}
+    steps:
+      - name: Determine whether this PR is the tip of its stack
+        id: detect
+        env:
+          GH_TOKEN: ${{ github.token }}
+          HEAD_REF: ${{ github.head_ref }}
+          REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+
+          # Count OPEN PRs that target this PR's head branch as their base.
+          # Any such PR means work is still stacked on top → not the tip.
+          children=$(gh pr list --repo "$REPO" --state open --base "$HEAD_REF" \
+            --json number --jq 'length' 2>/dev/null || echo 0)
+          children=${children:-0}
+
+          if [ "$children" -gt 0 ]; then
+            echo "is_tip=false" >> "$GITHUB_OUTPUT"
+            echo "This PR has $children open PR(s) stacked on top — changes still in flight."
+            echo "The OpenSpec archive check is skipped until this PR is the tip of the stack."
+          else
+            echo "is_tip=true" >> "$GITHUB_OUTPUT"
+            echo "No PRs are stacked on top — this PR is the tip (or standalone); the archive check will run."
+          fi
+
   check-openspec-archived:
+    needs: stack-position
+    if: needs.stack-position.outputs.is_tip == 'true'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
@@ -28,8 +66,8 @@ jobs:
             echo "::error::Unarchived OpenSpec change directories found under openspec/changes/:"
             echo "$UNARCHIVED" | sed 's|^|  - |'
             echo ""
-            echo "Every change must be moved under openspec/changes/archive/ before merge."
-            echo "Run the openspec archive workflow (e.g. /openspec-archive-change <name>) and commit the result."
+            echo "This PR is the tip of its stack (all work landed), so the change must be archived."
+            echo "Move it under openspec/changes/archive/ (e.g. /openspec-archive-change <name>) and commit."
             exit 1
           fi