Fix PlanAsync fallback and BeginTask TaskGraph conflict

Two runtime bugs discovered during MCP integration testing:

1. PlannerService.BuildTaskGraph fallback: If the LLM-generated plan
   contains invalid tasks (bad estimates, cycles, etc.), graph.AddTask
   throws PlannerValidationException. Previously this propagated up and
   caused "No tasks were executed." or "Planning failed" errors.
   Fix: wrap BuildTaskGraph in try/catch, fall back to single-task
   graph. Also handle graph.Count == 0 after filtering.

2. TaskGraphToolHandler.BeginTask conflict: PlanAsync stores the
   planned TaskGraph in session.TaskGraphs[taskId] before RunAsync
   calls BeginTaskAsync. BeginTask then tried TryAdd which failed
   because the key already existed, throwing
   "TaskGraph for '...' already exists in session".
   Fix: use TryAdd without throwing on conflict — if PlanAsync already
   created the graph, keep it; otherwise add a new empty one.

Test results: Core 61/61, Planner 10/10, Windows 41/41 (elevated).
MCP integration: agent_run/agent_status verified via HTTP transport.
ACL boundary: C:\Windows\System32 access correctly denied by sandbox.
ToolUseGuard: "Required external effect was not completed" fires
correctly when LLM does not produce verifiable external effect.

2 files changed, 26 insertions, 7 deletions

diff --git a/src/Agent.Core/Runtime/PlannerService.cs b/src/Agent.Core/Runtime/PlannerService.cs
index 7a450d0..110e476 100644
--- a/src/Agent.Core/Runtime/PlannerService.cs
+++ b/src/Agent.Core/Runtime/PlannerService.cs
@@ -55,7 +55,15 @@ public sealed class PlannerService(IOllamaClient ollama, AgentRuntimeSettings se
         if (plan is null || plan.Tasks.Count == 0)
             return CreateFallbackGraph(request);
 
-        TaskGraph graph = BuildTaskGraph(plan, request);
+        TaskGraph graph;
+        try
+        {
+            graph = BuildTaskGraph(plan, request);
+        }
+        catch
+        {
+            return CreateFallbackGraph(request);
+        }
         session.TaskGraphs[taskId] = graph;
         return graph;
     }
@@ -116,10 +124,21 @@ public sealed class PlannerService(IOllamaClient ollama, AgentRuntimeSettings se
             }
 
             string description = string.IsNullOrWhiteSpace(pt.Description) ? originalRequest : pt.Description;
-            PlannerTask task = graph.AddTask(description, budgetClass, estimate, dependencies, pt.RequiresExternalEffect);
-            idMap.Add(task.Id);
+            try
+            {
+                PlannerTask task = graph.AddTask(description, budgetClass, estimate, dependencies, pt.RequiresExternalEffect);
+                idMap.Add(task.Id);
+            }
+            catch (PlannerValidationException)
+            {
+                // If a task fails validation (e.g., bad estimate, cycle), fall back to single-task graph.
+                return CreateFallbackGraph(originalRequest);
+            }
         }
 
+        // If all tasks were filtered out, fall back.
+        if (graph.Count == 0) return CreateFallbackGraph(originalRequest);
+
         return graph;
     }
 
diff --git a/src/Agent.Core/Tools/TaskGraphToolHandler.cs b/src/Agent.Core/Tools/TaskGraphToolHandler.cs
index edc1cc5..10094d3 100644
--- a/src/Agent.Core/Tools/TaskGraphToolHandler.cs
+++ b/src/Agent.Core/Tools/TaskGraphToolHandler.cs
@@ -21,13 +21,13 @@ public sealed class TaskGraphToolHandler(int maxTasks = 32, SessionStore? sessio
         TaskGraph graph = new(maxTasks);
         if (sessions is not null && sessions.TryGet(sessionId, out AgentSession? session) && session is not null)
         {
-            if (!session.TaskGraphs.TryAdd(taskId, graph))
-                throw new InvalidOperationException($"TaskGraph for '{taskId}' already exists in session '{sessionId}'.");
+            // PlanAsync may have already created a TaskGraph for this taskId.
+            // If so, keep the planned graph; otherwise add a new empty one.
+            session.TaskGraphs.TryAdd(taskId, graph);
         }
         else
         {
-            if (!_fallbackGraphs.TryAdd(taskId, graph))
-                throw new InvalidOperationException($"TaskGraph for '{taskId}' already exists.");
+            _fallbackGraphs.TryAdd(taskId, graph);
         }
     }