P1b: Document read-only MCP tools as TaskBridgeService facade

Implementation verified (originally in aaec3eb):
- nexus_get_board: pure facade → bridge.GetBoardAsync()
- nexus_agent_overview: pure facade → bridge.GetAgentOverviewAsync()
- nexus_get_task: pure facade → bridge.GetTaskAsync() + ToResponse()
- nexus_get_children: pure facade → bridge.GetChildTasksAsync()
- nexus_get_activity: pure facade → bridge.GetTaskActivityAsync() + DTO map

All 5 tools register via [McpServerTool] attribute on McpServerToolType class.
No business logic duplication — every tool delegates directly to ITaskBridgeService.
Build: 0w 0e. MCP tests: 44/44 passed.

backend/Services/NexusMcpTools.cs

@@ -15,6 +15,8 @@ public sealed class NexusMcpTools(
     IConfiguration configuration,
     ILogger<NexusMcpTools> logger)
 {
+    // ── P1b: Read-only MCP Tools (TaskBridgeService facade) ──
+
     [McpServerTool(Name = "nexus_get_board")]
     [Description("Get the full Nexus task board grouped by canonical states.")]
     public async Task<BoardResponse> GetBoard(CancellationToken ct = default)
@@ -59,6 +61,8 @@ public sealed class NexusMcpTools(
         return activity.Select(entry => new ActivityEntryDto(entry.Id, entry.Type, entry.Message, entry.CreatedAt)).ToList();
     }
 
+    // ── P1c: Mutating MCP Tools ──
+
     [McpServerTool(Name = "nexus_create_task")]
     [Description("Create a top-level Nexus task.")]
     public async Task<TaskBridgeCommandResponse<DashboardTaskDto>> CreateTask(