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/sync.go

@@ -1045,9 +1045,9 @@ func showSyncStatus(ctx context.Context) error {
 		fmt.Print(string(logOutput))
 	}
 
-	// Show file diff for .beads/beads.jsonl
-	fmt.Println("\nFile differences in .beads/beads.jsonl:")
-	diffCmd := exec.CommandContext(ctx, "git", "diff", currentBranch+"..."+syncBranch, "--", ".beads/beads.jsonl")
+	// Show file diff for .beads/issues.jsonl
+	fmt.Println("\nFile differences in .beads/issues.jsonl:")
+	diffCmd := exec.CommandContext(ctx, "git", "diff", currentBranch+"..."+syncBranch, "--", ".beads/issues.jsonl")
 	diffOutput, err := diffCmd.CombinedOutput()
 	if err != nil {
 		// diff returns non-zero when there are differences, which is fine