Agent View: make PR label and row summary configurable (latest-PR option + work summary)

What

Two requests for the per-session rows in Agent View (claude agents):

1. Latest-PR option. When a session has multiple PRs, the row shows a count (e.g. 5 PRs). Add a setting to instead show the most recent PR (PR #1234). The latest PR is usually the one I care about; the count tells me little and hides the link.

2. Richer "what happened" summary. The auto Haiku one-line summary is great for current activity, but I'd also like the row (or the Space peek panel) to optionally summarize what the session did: distinct PRs opened, branch/worktree, artifacts produced. This data already exists in the session JSONL (pr-link, last-prompt, worktree-state, frame-link entries).

Why

As sessions accumulate work, the N PRs count column becomes the least useful thing in the row — I can't click it, and it doesn't tell me which PR is current. A configurable row would let me trade the count for latest-PR + a short work summary.

Suggestion

A settings.json option for Agent View row layout, e.g. choosing the PR display mode (count vs latest) and whether to surface a work summary in the row or peek panel.