Improve background agent visibility: sort active agents to top and show current activity

Problem

Two issues with monitoring background agents:

1. In Ctrl+O, completed agents bury the still-running ones. For example, if 5 agents are launched in parallel and 4 finish quickly, the one still running gets buried among the completed ones — regardless of whether it started at the same time or earlier.
2. There's no way to see what that remaining agent is actually doing. It could be stuck reading a long paper, waiting on a slow tool call, or hitting an error — but all you see is that it's still "running" with no further detail.

Current Behavior

• Ctrl+O shows all agents in a flat list ordered by launch time, not by status
• When 4 out of 5 parallel agents complete, the one still running gets lost in the list
• No way to peek at what the running agent is doing — it just appears stuck
• You're forced to either wait blindly or kill it without knowing if it's making progress

Proposed Solution

1. Sort/filter the agent list by status, not just launch order

• Always keep running/pending agents visible at the top (or in a dedicated section), regardless of when they were launched
• If 1 out of 5 agents is still running, it should be immediately obvious and easy to find

2. Show current activity of running agents

• Display what the agent is currently doing (e.g., "Reading file X — page 12/45", "Running bash command...", "Fetching URL...")
• Allow expanding a running agent in the Ctrl+O view to see its latest tool call or output
• This lets users distinguish between "stuck" and "working on something large" and decide whether to wait or intervene

Use Case

Launched 5 background agents in parallel. 4 completed quickly. The 5th is probably reading a long paper and taking a while, but it appears stuck because there's no visibility into its progress. I can't tell if it's making progress, hit an error, or is genuinely stuck. I have to either wait blindly or kill it and lose the work. Being able to quickly find it in the list and see what it's doing would solve this.