fix(help): prevent /help tab crash from undefined descriptions (#732)

- Guard formatDescriptionWithSource() so missing command descriptions become ''
- Harden truncate helpers to accept undefined text/path safely
- Add regression tests covering undefined input cases

src/commands.ts

@@ -740,23 +740,23 @@ export function getCommand(commandName: string, commands: Command[]): Command {
  */
 export function formatDescriptionWithSource(cmd: Command): string {
   if (cmd.type !== 'prompt') {
-    return cmd.description
+    return cmd.description ?? ''
   }
 
   if (cmd.kind === 'workflow') {
-    return `${cmd.description} (workflow)`
+    return `${cmd.description ?? ''} (workflow)`
   }
 
   if (cmd.source === 'plugin') {
     const pluginName = cmd.pluginInfo?.pluginManifest.name
     if (pluginName) {
-      return `(${pluginName}) ${cmd.description}`
+      return `(${pluginName}) ${cmd.description ?? ''}`
     }
-    return `${cmd.description} (plugin)`
+    return `${cmd.description ?? ''} (plugin)`
   }
 
   if (cmd.source === 'builtin' || cmd.source === 'mcp') {
-    return cmd.description
+    return cmd.description ?? ''
   }
 
   if (cmd.source === 'bundled') {