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/merge.go

@@ -93,7 +93,7 @@ Vendored into bd with permission.`,
 
 func cleanupMergeArtifacts(outputPath string, debug bool) {
 	// Determine the .beads directory from the output path
-	// outputPath is typically .beads/beads.jsonl
+	// outputPath is typically .beads/issues.jsonl
 	beadsDir := filepath.Dir(outputPath)
 
 	if debug {