fix(skills): restore worktree step in brainstorming workflow

The brainstorming skill never invoked using-git-worktrees despite
writing-plans expecting to run in a worktree "created by brainstorming
skill" and using-git-worktrees listing brainstorming as a caller.

This was unintentionally dropped during workflow simplification
(8e38ab8, 7f2ee61) and remained missing after v5.0.0 rework.

Changes to skills/brainstorming/SKILL.md:
- Checklist: insert step 7 (Set up worktree) between design doc and
  writing-plans
- Process flow diagram: add worktree node with correct edges
- Terminal state note: update skill whitelist to include
  using-git-worktrees
- After the Design: add Workspace isolation section, adjust
  Implementation prohibition from "any other skill" to
  "any implementation skill (frontend-design, mcp-builder, etc.)"

Changes to skills/using-git-worktrees/SKILL.md:
- Integration: fix stale "Phase 4" reference to "Step 7"

Closes #574
Ref #186

Author: qishaoyumu

skills/brainstorming/SKILL.md

@@ -29,7 +29,8 @@ You MUST create a task for each of these items and complete them in order:
 6. **Write design doc** — save to `docs/superpowers/specs/YYYY-MM-DD-<topic>-design.md` and commit
 7. **Spec review loop** — dispatch spec-document-reviewer subagent; fix issues and re-dispatch until approved (max 5 iterations, then surface to human)
 8. **User reviews written spec** — ask user to review the spec file before proceeding
-9. **Transition to implementation** — invoke writing-plans skill to create implementation plan
+9. **Set up worktree** — invoke using-git-worktrees to create isolated implementation workspace
+10. **Transition to implementation** — invoke writing-plans skill to create implementation plan
 
 ## Process Flow
 
@@ -46,6 +47,7 @@ digraph brainstorming {
     "Spec review loop" [shape=box];
     "Spec review passed?" [shape=diamond];
     "User reviews spec?" [shape=diamond];
+    "Set up worktree\n(using-git-worktrees)" [shape=box];
     "Invoke writing-plans skill" [shape=doublecircle];
 
     "Explore project context" -> "Visual questions ahead?";
@@ -62,11 +64,12 @@ digraph brainstorming {
     "Spec review passed?" -> "Spec review loop" [label="issues found,\nfix and re-dispatch"];
     "Spec review passed?" -> "User reviews spec?" [label="approved"];
     "User reviews spec?" -> "Write design doc" [label="changes requested"];
-    "User reviews spec?" -> "Invoke writing-plans skill" [label="approved"];
+    "User reviews spec?" -> "Set up worktree\n(using-git-worktrees)" [label="approved"];
+    "Set up worktree\n(using-git-worktrees)" -> "Invoke writing-plans skill";
 }
 ```
 
-**The terminal state is invoking writing-plans.** Do NOT invoke frontend-design, mcp-builder, or any other implementation skill. The ONLY skill you invoke after brainstorming is writing-plans.
+**The terminal state is invoking writing-plans.** Do NOT invoke frontend-design, mcp-builder, or any other implementation skill. The ONLY skills you invoke after brainstorming are using-git-worktrees (for workspace isolation) then writing-plans.
 
 ## The Process
 
@@ -130,10 +133,15 @@ After the spec review loop passes, ask the user to review the written spec befor
 
 Wait for the user's response. If they request changes, make them and re-run the spec review loop. Only proceed once the user approves.
 
+**Workspace isolation:**
+
+- Invoke using-git-worktrees to create an isolated worktree for implementation
+- This ensures implementation work doesn't affect the main working tree
+
 **Implementation:**
 
 - Invoke the writing-plans skill to create a detailed implementation plan
-- Do NOT invoke any other skill. writing-plans is the next step.
+- Do NOT invoke any implementation skill (frontend-design, mcp-builder, etc.). writing-plans is the next step after workspace isolation.
 
 ## Key Principles
 

skills/using-git-worktrees/SKILL.md

@@ -209,7 +209,7 @@ Ready to implement auth feature
 ## Integration
 
 **Called by:**
-- **brainstorming** (Phase 4) - REQUIRED when design is approved and implementation follows
+- **brainstorming** (Step 9) - REQUIRED after user approves spec, before invoking writing-plans
 - **subagent-driven-development** - REQUIRED before executing any tasks
 - **executing-plans** - REQUIRED before executing any tasks
 - Any skill needing isolated workspace