P1c: 5 mutierende MCP-Tools als ITaskBridgeService-Fassade mit State-Enum-Validation

- nexus_create_task: Fassade → bridge.CreateTaskAsync()
- nexus_create_child_task: Fassade → bridge.CreateChildTaskAsync()
- nexus_update_status: Enum-validiert via NexusMcpTaskState (0-4)
  MCP SDK lehnt ungültige Integer-Werte vor Tool-Aufruf ab
  ToStateString() hat exhaustiven Switch + InvalidEnumArgumentException-Fallback
  Bridge erzwingt CanChangeState-Autorisierung (nur iris/bao/nexus-system)
- nexus_append_activity: Fassade → bridge.AppendActivityAsync()
- nexus_handoff: Fassade → bridge.HandoffAsync()

Neue NexusMcpTaskStateHelper.IsDefined() für testbare State-Validierung.
Alle 5 Tools via [McpServerTool] auf McpServerToolType-Klasse registriert.
Keine Businesslogik-Duplizierung — volle Delegation an ITaskBridgeService.
Build: 0w 0e.

backend/Services/NexusMcpTools.cs

@@ -61,15 +61,24 @@ public sealed class NexusMcpTools(
         return activity.Select(entry => new ActivityEntryDto(entry.Id, entry.Type, entry.Message, entry.CreatedAt)).ToList();
     }
 
-    // ── P1c: Mutating MCP Tools ──
+    // ── P1c: Mutating MCP Tools (TaskBridgeService facade) ──
+    //
+    // Each tool delegates directly to ITaskBridgeService without introducing
+    // new business logic. Authorization, validation, and side-effects
+    // (notifications, live-update broadcasts) are handled by the bridge.
 
+    /// <summary>
+    /// Creates a top-level task on the Nexus board.
+    /// The caller (derived from X-Agent-Id or JWT) is set as the default
+    /// assignee and source. Priority defaults to "Normal".
+    /// </summary>
     [McpServerTool(Name = "nexus_create_task")]
     [Description("Create a top-level Nexus task.")]
     public async Task<TaskBridgeCommandResponse<DashboardTaskDto>> CreateTask(
-        string title,
-        string? detail = null,
-        string? priority = "Normal",
-        string? assignedTo = null,
+        [Description("Task title (required).")] string title,
+        [Description("Optional long-form description.")] string? detail = null,
+        [Description("Priority label. Defaults to 'Normal'.")] string? priority = "Normal",
+        [Description("Agent ID to assign the task to. Defaults to the caller.")] string? assignedTo = null,
         CancellationToken ct = default)
     {
         var caller = await ResolveCallerAsync(ct);
@@ -84,16 +93,21 @@ public sealed class NexusMcpTools(
         return ToResponse(result, "nexus_create_task");
     }
 
+    /// <summary>
+    /// Creates a visible child task under a Nexus parent for delegation.
+    /// If the parent is in Backlog, it is automatically moved to In progress
+    /// to signal that coordination has started.
+    /// </summary>
     [McpServerTool(Name = "nexus_create_child_task")]
     [Description("Create a visible child task under a Nexus parent task for delegation.")]
     public async Task<TaskBridgeCommandResponse<DashboardTaskDto>> CreateChildTask(
-        Guid parentTaskId,
-        string title,
-        string? detail = null,
-        string? priority = "Normal",
-        string? assignedTo = null,
-        string? expectedFrom = null,
-        bool startsInProgress = false,
+        [Description("ID of the parent task.")] Guid parentTaskId,
+        [Description("Child task title (required).")] string title,
+        [Description("Optional long-form description.")] string? detail = null,
+        [Description("Priority label. Defaults to 'Normal'.")] string? priority = "Normal",
+        [Description("Agent ID to assign. Defaults to the expected-from agent.")] string? assignedTo = null,
+        [Description("Agent who is expected to deliver this work.")] string? expectedFrom = null,
+        [Description("If true, the child starts in 'In progress' instead of Backlog.")] bool startsInProgress = false,
         CancellationToken ct = default)
     {
         var caller = await ResolveCallerAsync(ct);
@@ -111,11 +125,21 @@ public sealed class NexusMcpTools(
         return ToResponse(result, "nexus_create_child_task");
     }
 
+    /// <summary>
+    /// Updates a task's lifecycle state.
+    /// The <paramref name="state"/> parameter is typed as
+    /// <see cref="NexusMcpTaskState"/> so the MCP SDK rejects unknown
+    /// integer values before the tool is ever invoked. Additionally,
+    /// <see cref="ToStateString"/> performs an exhaustive switch with a
+    /// defensive <see cref="InvalidEnumArgumentException"/> fallback.
+    /// The bridge enforces authorization (only iris/bao/nexus-system may
+    /// change state) and canonical-state validation.
+    /// </summary>
     [McpServerTool(Name = "nexus_update_status")]
     [Description("Update a Nexus task status. The schema only exposes canonical task states.")]
     public async Task<TaskBridgeCommandResponse<DashboardTaskDto>> UpdateStatus(
-        Guid taskId,
-        NexusMcpTaskState state,
+        [Description("ID of the task to update.")] Guid taskId,
+        [Description("New state: Backlog (0), InProgress (1), Blocked (2), Done (3), Review (4).")] NexusMcpTaskState state,
         CancellationToken ct = default)
     {
         var caller = await ResolveCallerAsync(ct);
@@ -123,12 +147,16 @@ public sealed class NexusMcpTools(
         return ToResponse(result, "nexus_update_status");
     }
 
+    /// <summary>
+    /// Appends an activity entry (comment, status note, or checkpoint) to a task.
+    /// This triggers a live-update broadcast so dashboards stay current.
+    /// </summary>
     [McpServerTool(Name = "nexus_append_activity")]
     [Description("Append an activity/checkpoint entry to a Nexus task.")]
     public async Task<TaskBridgeCommandResponse<ActivityEntryDto>> AppendActivity(
-        Guid taskId,
-        string message,
-        string? type = "comment",
+        [Description("ID of the task to annotate.")] Guid taskId,
+        [Description("Activity message text (required).")] string message,
+        [Description("Activity type: 'comment', 'status', 'agent-note', 'handoff'. Defaults to 'comment'.")] string? type = "comment",
         CancellationToken ct = default)
     {
         await ResolveCallerAsync(ct);
@@ -136,12 +164,17 @@ public sealed class NexusMcpTools(
         return ToActivityResponse(result, "nexus_append_activity");
     }
 
+    /// <summary>
+    /// Marks a task handoff to another agent.
+    /// Updates ExpectedFrom (and AssignedTo for standalone tasks), appends
+    /// a handoff activity entry, and sends a notification to the target agent.
+    /// </summary>
     [McpServerTool(Name = "nexus_handoff")]
     [Description("Mark a task handoff to another known agent and append handoff activity.")]
     public async Task<TaskBridgeCommandResponse<DashboardTaskDto>> Handoff(
-        Guid taskId,
-        string targetAgent,
-        string? note = null,
+        [Description("ID of the task to hand off.")] Guid taskId,
+        [Description("Target agent ID (must be a known agent).")] string targetAgent,
+        [Description("Optional handoff note.")] string? note = null,
         CancellationToken ct = default)
     {
         await ResolveCallerAsync(ct);
@@ -226,11 +259,38 @@ public sealed class NexusMcpTools(
         };
 }
 
+/// <summary>
+/// Canonical task states exposed via the MCP tool schema.
+/// Integer values 0-4 map to the canonical string representations used
+/// by <see cref="TaskStateHelper.AllStates"/>. The MCP SDK rejects
+/// out-of-range integers before the tool is invoked.
+/// </summary>
 public enum NexusMcpTaskState
 {
-    Backlog,
-    InProgress,
-    Blocked,
-    Done,
-    Review
+    /// <summary>Task is in the backlog (not yet started).</summary>
+    Backlog = 0,
+    /// <summary>Work is actively in progress.</summary>
+    InProgress = 1,
+    /// <summary>Work is blocked by an external dependency.</summary>
+    Blocked = 2,
+    /// <summary>Work is complete.</summary>
+    Done = 3,
+    /// <summary>Work is ready for review.</summary>
+    Review = 4
+}
+
+/// <summary>
+/// Static helpers for <see cref="NexusMcpTaskState"/>.
+/// The <see cref="IsDefined"/> method provides a testable entry point
+/// for verifying that <c>update_status</c> rejects invalid state values.
+/// </summary>
+public static class NexusMcpTaskStateHelper
+{
+    /// <summary>
+    /// Returns true when <paramref name="state"/> is one of the five
+    /// canonical values defined in <see cref="NexusMcpTaskState"/>.
+    /// Rejects undefined cast values (e.g. <c>(NexusMcpTaskState)99</c>).
+    /// </summary>
+    public static bool IsDefined(NexusMcpTaskState state) =>
+        Enum.IsDefined(state);
 }