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>

CONTRIBUTING.md

@@ -113,6 +113,18 @@ Add cycle detection for dependency graphs
 - Update documentation with examples
 ```
 
+### Important: Don't Include .beads/issues.jsonl Changes
+
+The `.beads/issues.jsonl` file is the project's issue database. **Do not include changes to this file in your PR.** CI will fail if this file is modified.
+
+If you accidentally committed changes to this file, fix it with:
+
+```bash
+git checkout origin/main -- .beads/issues.jsonl
+git commit --amend
+git push --force
+```
+
 ### Pull Requests
 
 - Keep PRs focused on a single feature or fix
@@ -120,6 +132,7 @@ Add cycle detection for dependency graphs
 - Update documentation as needed
 - Ensure CI passes before requesting review
 - Respond to review feedback promptly
+- **Do not include `.beads/issues.jsonl` changes** (see above)
 
 ## Testing Guidelines