Mark conversation as resolved without sending a message

Problem

When a session is effectively finished, the sidebar still shows the activity indicator (the dot/icon on the left of the session) as if the conversation has an active or pending task. There is no explicit UI for closing out a thread.

To clear that indicator today, the user has to send a message like "done" so the model recognizes the conversation is complete and stops. This works, but it is wasteful — it sends a full prompt + tool-use turn just to close the thread, which burns tokens (and sometimes the model still continues working instead of stopping).

<img width="3508" height="2086" alt="Image" src="https://github.com/user-attachments/assets/b8d762cd-ad53-46f2-b638-0c8fcaddb43d" />

<img width="3508" height="2086" alt="Image" src="https://github.com/user-attachments/assets/1d039685-52cf-4608-95f5-73417ea5c435" />

Proposed solution

Add a simple, explicit way to mark a session as "resolved" / "done" — for example:

• A button or menu item on the session (e.g. right-click on the sidebar entry → "Mark as done")
• A keyboard shortcut while the session is focused
• Possibly an explicit slash command like /done that the harness handles locally (no model turn)

The effect should be:

• The activity indicator next to the session disappears
• No new model turn is created (no tokens spent)
• The session stays in the sidebar, just no longer shown as "active"

Why it matters

• Token cost: closing threads with a real prompt sends a turn for no real work
• Latency / friction: waiting for the model to acknowledge "done" is slower than a one-click UI action
• Clarity: the sidebar should distinguish between "this session has unfinished work" and "this session is parked / complete". Right now the indicator means both.

Reproduction

1. Open any session, complete the work
2. Notice the activity indicator on the session in the sidebar
3. The only way to clear it is to type into the prompt (e.g. "done") and wait for the model to finish

Expected

Be able to mark the session as done from the UI directly, with no model round-trip