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.test.ts

@@ -0,0 +1,30 @@
+import { formatDescriptionWithSource } from './commands.js'
+
+describe('formatDescriptionWithSource', () => {
+  test('returns empty text for prompt commands missing a description', () => {
+    const command = {
+      name: 'example',
+      type: 'prompt',
+      source: 'builtin',
+      description: undefined,
+    } as any
+
+    expect(formatDescriptionWithSource(command)).toBe('')
+  })
+
+  test('formats plugin commands with missing description safely', () => {
+    const command = {
+      name: 'example',
+      type: 'prompt',
+      source: 'plugin',
+      description: undefined,
+      pluginInfo: {
+        pluginManifest: {
+          name: 'MyPlugin',
+        },
+      },
+    } as any
+
+    expect(formatDescriptionWithSource(command)).toBe('(MyPlugin) ')
+  })
+})