feat(update): add --append-notes flag (#1304)

* feat(update): add --append-notes flag (bd-b5qu)

Add --append-notes flag that appends to existing notes with a newline
separator instead of overwriting. This prevents data loss in workflows
where multiple steps need to add info to notes (e.g., tackle workflows).

- Errors if both --notes and --append-notes specified
- Handles both daemon and direct mode paths
- Combines existing notes + newline + new content

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* test(update): add tests for --append-notes flag

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/flags.go

@@ -22,6 +22,7 @@ func registerCommonIssueFlags(cmd *cobra.Command) {
 	cmd.Flags().String("design", "", "Design notes")
 	cmd.Flags().String("acceptance", "", "Acceptance criteria")
 	cmd.Flags().String("notes", "", "Additional notes")
+	cmd.Flags().String("append-notes", "", "Append to existing notes (with newline separator)")
 	cmd.Flags().String("external-ref", "", "External reference (e.g., 'gh-9', 'jira-ABC')")
 }