Fix: Change default JSONL filename from beads.jsonl to issues.jsonl

The canonical beads database name is issues.jsonl. Tens of thousands of users
have issues.jsonl, and beads.jsonl was only used by the Beads project itself
due to git history pollution.

Changes:
- Updated bd doctor to warn about beads.jsonl instead of issues.jsonl
- Changed default config from beads.jsonl to issues.jsonl
- Reversed precedence in checkGitForIssues to prefer issues.jsonl
- Updated git merge driver config to use issues.jsonl
- Updated all tests to expect issues.jsonl as the default

issues.jsonl is now the canonical default; beads.jsonl is legacy

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

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

cmd/bd/merge.go

@@ -28,7 +28,7 @@ Designed to work as a git merge driver. Configure with:
 
   git config merge.beads.driver "bd merge %A %O %A %B"
   git config merge.beads.name "bd JSONL merge driver"
-  echo ".beads/beads.jsonl merge=beads" >> .gitattributes
+  echo ".beads/issues.jsonl merge=beads" >> .gitattributes
 
 Or use 'bd init' which automatically configures the merge driver.