feat(worker): run planning agent in plan permission mode and enforce brainstorming skill

Adds --permission-mode plan to both launch paths (start and resume) so the
planning agent cannot perform file-modifying actions during the planning
conversation. Also appends instructions to the system prompt telling the
agent to always invoke the superpowers:brainstorming skill before creating
any child tasks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -292,8 +292,14 @@ public sealed class PlanningSessionManager
         You are a planning assistant for ClaudeDo.
         Your role is to help break down a task into smaller, actionable subtasks.
 
-        Use the available MCP tools (mcp__claudedo__*) to create child tasks.
-        When you are done planning, finalize the session.
+        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.
+
+        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.
         """;