ci: add check to reject PRs with .beads/issues.jsonl changes

Contributors frequently fork the repo and accidentally include their local
.beads/issues.jsonl changes in PRs. This adds:

1. A new CI job that fails PRs containing .beads/issues.jsonl changes
2. Clear error message with fix instructions
3. Updated CONTRIBUTING.md with guidance

This should prevent the common issue of PRs including unintended database changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -7,6 +7,32 @@ on:
     branches: [ main ]
 
 jobs:
+  # Fast check to catch accidental .beads/issues.jsonl changes from contributors
+  check-no-beads-changes:
+    name: Check for .beads changes
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Check for .beads/issues.jsonl changes
+        run: |
+          if git diff --name-only origin/${{ github.base_ref }}...HEAD | grep -q "^\.beads/issues\.jsonl$"; then
+            echo "❌ This PR includes changes to .beads/issues.jsonl"
+            echo ""
+            echo "This file is the project's issue database and should not be modified in PRs."
+            echo ""
+            echo "To fix, run:"
+            echo "  git checkout origin/main -- .beads/issues.jsonl"
+            echo "  git commit --amend"
+            echo "  git push --force"
+            echo ""
+            exit 1
+          fi
+          echo "✅ No .beads/issues.jsonl changes detected"
+
   test:
     name: Test
     runs-on: ubuntu-latest