Fleet view: show live agent activity across channels and mobile

**Motivation**

Buzz knows when my agents are working, but I can't see that work in one place.

On desktop, the left rail shows timers. To understand what an agent is doing, I have to return to the originating conversation, hover over the composer activity indicator, then open **View activity**. With agents working across several channels or DMs, I have to repeat that process for each one.

Mobile is more confusing. The app has an **Activity** screen and an **Activity** filter, but these refer to recent channel activity. It can show `Activity (0)` and "No recent channel activity" while agent work is only available through the desktop's per-conversation activity surface.

"Activity" currently means channel notifications on mobile, published content in Pulse, and live agent execution in the desktop panel. None of those surfaces answers the workspace-level question: what are my agents doing right now?

**Proposed solution**

Add a workspace-wide agent activity view on desktop and mobile showing:

- Every owned agent currently working
- A concise description of the current action
- The originating channel, DM or thread
- Elapsed time and latest update
- Working, waiting, blocked and failed states
- One-click access to the existing activity log
- One-click navigation back to the source conversation

The first slice can be read-only and reuse the existing observer events, working signals and session panel. It doesn't need new transcript UI, usage analytics or remote lifecycle controls.

I'd also relabel the existing mobile filter as **Channel activity** so it doesn't imply that it shows agent execution.

**Alternatives considered**

- Composer activity pills are useful inside one conversation, but require finding every source conversation individually.
- Pulse is currently a publishing and social feed rather than a live operational view.
- The Agents page shows the roster and configuration, but not what each agent is currently doing.
- A separate external dashboard would duplicate data Buzz already receives and weaken the native orchestration experience.

**Additional context**

No exact open issue was found, but there is strong prior work:

- [#3684](https://github.com/block/buzz/pull/3684) implemented an Agent Fleet view with live status, source channels, elapsed time and click-through to the existing activity panel. The author closed the draft shortly after opening it, without review or a maintainer decision.
- [#2716](https://github.com/block/buzz/issues/2716) includes another contributor building a fleet view from observer events and presence.
- [#2646](https://github.com/block/buzz/pull/2646) implements the current per-conversation composer activity pills.
- [#3051](https://github.com/block/buzz/issues/3051) proposes a persistent full-screen live session view.
- [#3907](https://github.com/block/buzz/issues/3907) reports the missing agent-observability path on mobile.
- [#3996](https://github.com/block/buzz/issues/3996) proposes an at-a-glance Agents surface for remote agents.

Before starting another implementation, would maintainers prefer #3684 to be revived and narrowed, or a fresh first slice against current `main`?

Mobile screenshot attached showing the current **Activity (0)** and "No recent channel activity" state.

Review prior work in #3684 and #2716 first, then trace the existing observer events, working signals, session panel, and mobile Activity filter described in the issue. Done means a narrowed first slice shows owned agents, current action, source, elapsed time, update and state on desktop and mobile, with links to the existing activity log and source conversation.

Written by the indexing model from the issue text.