Feature Request: Allow the board to be visible while chatting with agents

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I am using a recent [release](https://github.com/block/berd/releases) and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Closest existing issue

None found

### Is this new, or an improvement?

New capability — Berd can't do this at all today

### The problem, in your terms

The board is what drew me to Berd in the first place. Having agents, notes, and projects in a spatial layout feels really fresh and fun, and helps me way more than I thought with staying organised and keeping track of stuff I'm working on.

Right now, start a chat with an agent present on the board takes over the whole screen with a standard 1:1 chat window and hides the board itself. Losing that spatial context feels like a context switch, and makes it harder to stay aware of what multiple agents are doing, what projects are where, etc.

### What you do today

Just clicking back and forth between the chat view and the board with the mouse.

### What you'd like to see

Richer interactions and presence of the agents pinned to the board so they can both have visibility of their current "status" from the context of the board and also be interacted with without having to leave the board.

For example, if I give Agent A a big task that takes a few minutes, I'd love to see a little thinking animation on them on the board while I start chatting with Agent B. When Agent A finishes, seeing a speech bubble or status update right there on the canvas would keep me in the flow, feel more efficient, and lean into the personality you've already built into the agents. Of course, being able to quickly and easily take that into a full-sized chat would be necessary I'm not suggesting a full blown replacement for the current pattern.

Right now agents pinned to the board kind of function more like shortcuts I think it's fair to say, there's no strong link between the little guy on your board and what an instance of that same agent might be doing.

### Why this belongs in Berd itself

The canvas and agent layout are core to how Berd looks and feels, so this needs to be part of the main app UI rather than a skill or extension.

### Non-goals

Not asking for complex visual scripting, node connectors, or board/canvas overhauls, just the ability to chat with agents without losing sight of the board, plus simple status cues (like ... when thinking, or an indicator when they finish).

### Alternatives you considered

_No response_

### Mockups, prior art, or other context

Start by reviewing the board and current full-screen chat interaction described in the issue. Define how chatting can remain visible alongside the board, including agent thinking and completion status cues. Done means users can chat without losing the board context and can still open a full-sized chat when needed.

Written by the indexing model from the issue text.