Background agents: parallel builds, visible models, honest statuses

Background agents got a big round of polish the same day they shipped:

• Agents now build in parallel. The AI can run several build agents at once - each with its own slice of the work - instead of queueing them one after another. Big builds finish sooner.
• You can see what each task runs on. Every task now shows its agent type and model tier right in the tasks panel, and read-only helpers (research, code exploration) always use the economy model. A task never uses a more expensive tier than your chat is set to - upgrading is always your call, never the AI's.
• Research you can trust. A research agent must fetch real web sources now; if it ever answers without them, the result is clearly flagged so you know it came from memory, not the web.
• Smoother task navigation. The tasks panel has a proper header and close button, and the back button in a task returns you to wherever you came from - the chat or the list. Failed and stopped tasks say what actually happened instead of a generic line.
• Calmer credit warnings. Running low shows a quiet amber note above the chat box (dismissible), with a heads-up at 50% and 15% remaining - including on the Free plan, which previously got no warning at all. Out of credits is now a one-click "Top up" (or "Upgrade" on Free), never a red alarm.
• Clearer controls. Stopping a task is a stop square now (the old ✕ looked like "close"), task cards keep their title readable on narrow screens, your chat list shows a small spinner on any chat the AI is still working in, and undo works on file changes inside a task's transcript too.