fix(delegate-task): validate description parameter and handle undefined in notifications

OpenCode's fromPlugin wrapper skips Zod validation for plugin tools, so
LLMs can omit required args like description without getting an error.
When Atlas orchestrates and the model omits description, it flows through
as undefined to manager.launch() and background task notifications show
'undefined' for all completed tasks.

Two fixes:
- Add runtime validation for description in delegate-task tool (matches
  existing run_in_background and load_skills validation pattern)
- Defensive fallback in notification template: use task ID when
  description is missing instead of rendering 'undefined'

src/tools/delegate-task/tools.ts

@@ -122,6 +122,9 @@ export function createDelegateTask(options: DelegateTaskToolOptions): ToolDefini
         title: args.description,
       })
 
+      if (!args.description || typeof args.description !== "string") {
+        throw new Error(`Invalid arguments: 'description' parameter is REQUIRED. Provide a short (3-5 words) task description.`)
+      }
       if (args.run_in_background === undefined) {
         throw new Error(`Invalid arguments: 'run_in_background' parameter is REQUIRED. Specify run_in_background=false for task delegation, or run_in_background=true for parallel exploration.`)
       }