feat(desktop): clarify agent activity header

[tellaho]

Category: improvement
User Impact: Activity feeds now clearly identify the agent and keep update recency visible even when channel names are long.

Problem: The activity header led with a generic label, making it hard to tell which agent was in view, while channel scope and recency competed for limited horizontal space. Long channel names could hide the update timestamp entirely.

Solution: Lead with the resolved agent avatar and name, then place mode and scope in a truncating metadata region with recency pinned at the right edge. This preserves the compact two-line header while keeping the most important identity and freshness signals legible.

File changes

desktop/src/features/channels/ui/AgentSessionThreadPanel.tsx
Reorganizes the activity header around the agent identity, reuses the existing resolved profile avatar and label helpers, and separates scope truncation from the always-visible recency label.

desktop/tests/e2e/activity-scope-label-screenshots.spec.ts
Expands activity-header coverage across channel-scoped, all-channel, raw, long-name, and narrow layouts, including measured truncation and recency visibility.

Reproduction steps

1. Open an agent's activity feed from a channel.
2. Confirm the agent avatar and name lead the header.
3. Open a feed scoped to a channel with a long name and resize the panel narrowly.
4. Confirm the mode and channel scope truncate while the recency label remains visible at the right edge.
5. Toggle Raw mode and open an all-channel feed to confirm the same hierarchy and truncation behavior.

Screenshots

Long channel	Narrow layout

Raw mode	All channels