refactor(prompts): planning prompts read from editable files

src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -363,47 +363,14 @@ public sealed class PlanningSessionManager
         }
         """;
 
-    private static string BuildSystemPrompt()
-    {
-        var fromFile = PromptFiles.ReadOrNull(PromptKind.Planning);
-        if (fromFile is not null) return fromFile;
+    private static string BuildSystemPrompt() => PromptFiles.ReadOrDefault(PromptKind.Planning);
 
-        return
-            """
-            You are a planning assistant for ClaudeDo.
-            Your role is to help break down a task into smaller, actionable subtasks.
-            Your final goal WILL ALWAYS be the creation of Subtasks
-
-            ALWAYS invoke the `superpowers:brainstorming` skill via the Skill tool at the
-            start of every planning session, and follow its process end-to-end. It guides
-            you through clarifying questions, approach exploration, and design approval
-            BEFORE any subtasks are created. Do not create child tasks until the user has
-            approved a design.
-
-            NEVER Change files yourself.
-
-            ALWAYS Use the available MCP tools (mcp__claudedo__*) to create child tasks once the
-            design is approved. When you are done planning, finalize the session.
-
-            Be concise and focused. Each subtask should be independently executable.
-            """;
-    }
-
-    private static string BuildInitialPrompt(TaskEntity task)
-    {
-        var sb = new StringBuilder();
-        sb.AppendLine($"# Task: {task.Title}");
-        if (!string.IsNullOrWhiteSpace(task.Description))
+    private static string BuildInitialPrompt(TaskEntity task) =>
+        PromptFiles.Render(PromptKind.PlanningInitial, new Dictionary<string, string>
         {
-            sb.AppendLine();
-            sb.AppendLine(task.Description);
-        }
-        sb.AppendLine();
-        sb.AppendLine("---");
-        sb.AppendLine();
-        sb.AppendLine("Please analyze this task and break it down into concrete subtasks.");
-        return sb.ToString();
-    }
+            ["title"] = task.Title,
+            ["description"] = task.Description ?? "",
+        });
 
     private static string BranchNameFor(string taskId) =>
         $"claudedo/planning/{taskId.Replace("-", "")}";