bd-6xd: Standardize on issues.jsonl as canonical filename

- Change default JSONL filename from beads.jsonl to issues.jsonl
- Add bd doctor check and fix to auto-migrate legacy beads.jsonl configs
- Update FindJSONLPath to prefer issues.jsonl over beads.jsonl
- Add CheckLegacyJSONLConfig and CheckLegacyJSONLFilename checks
- Add LegacyJSONLConfig fix to rename files and update config
- Update .gitattributes to reference issues.jsonl
- Fix tests to expect new canonical filename
- Add bd-6xd to v0.25.1 release notes

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

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

cmd/bd/cleanup.go

@@ -16,7 +16,7 @@ var cleanupCmd = &cobra.Command{
 	Short: "Delete closed issues from database to free up space",
 	Long: `Delete closed issues from the database to reduce database size.
 
-This command permanently removes closed issues from beads.db and beads.jsonl.
+This command permanently removes closed issues from beads.db and issues.jsonl.
 It does NOT remove temporary files - use 'bd clean' for that.
 
 By default, deletes ALL closed issues. Use --older-than to only delete