feat(notes): add_note MCP tool with a per-day agent cap

Agents can write into the user's daily notes, but only within limits:
max 10 agent notes a day, max 500 characters, add only — no edit, no
delete. A new from_agent column separates their lines from the user's
so the cap counts the right ones and the UI can mark them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ClaudeDo.Worker/Hub/HubBroadcaster.cs

@@ -37,6 +37,10 @@ public sealed class HubBroadcaster : IPrimeBroadcaster, IRefineBroadcaster
     public Task HandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds, string nextPhase) =>
         _hub.Clients.All.SendAsync("HandoffRequested", taskId, survivingTaskIds, nextPhase);
 
+    /// <summary>An agent wrote a note through MCP — the open notes editor reloads.</summary>
+    public Task NotesUpdated() =>
+        _hub.Clients.All.SendAsync("NotesUpdated");
+
     public Task ListUpdated(string listId) =>
         _hub.Clients.All.SendAsync("ListUpdated", listId);