fix(brainstorming): add user review gate between spec and writing-plans

After the spec review loop passes, the skill now asks the user to review
the written spec file before invoking writing-plans. This prevents the
agent from racing ahead to implementation planning without giving the
user a chance to read and adjust the written document.

Fixes #565

skills/brainstorming/SKILL.md

@@ -27,7 +27,8 @@ You MUST create a task for each of these items and complete them in order:
 4. **Propose 2-3 approaches** — with trade-offs and your recommendation
 5. **Present design** — in sections scaled to their complexity, get user approval after each section
 6. **Write design doc** — save to `docs/superpowers/specs/YYYY-MM-DD-<topic>-design.md` and commit
-7. **Transition to implementation** — invoke writing-plans skill to create implementation plan
+7. **User reviews written spec** — ask user to review the spec file before proceeding
+8. **Transition to implementation** — invoke writing-plans skill to create implementation plan
 
 ## Process Flow
 
@@ -41,6 +42,7 @@ digraph brainstorming {
     "Present design sections" [shape=box];
     "User approves design?" [shape=diamond];
     "Write design doc" [shape=box];
+    "User reviews spec?" [shape=diamond];
     "Invoke writing-plans skill" [shape=doublecircle];
 
     "Explore project context" -> "Visual questions ahead?";
@@ -52,7 +54,9 @@ digraph brainstorming {
     "Present design sections" -> "User approves design?";
     "User approves design?" -> "Present design sections" [label="no, revise"];
     "User approves design?" -> "Write design doc" [label="yes"];
-    "Write design doc" -> "Invoke writing-plans skill";
+    "Write design doc" -> "User reviews spec?";
+    "User reviews spec?" -> "Write design doc" [label="changes requested"];
+    "User reviews spec?" -> "Invoke writing-plans skill" [label="approved"];
 }
 ```
 
@@ -113,6 +117,13 @@ After writing the spec document:
 2. If Issues Found: fix, re-dispatch, repeat until Approved
 3. If loop exceeds 5 iterations, surface to human for guidance
 
+**User Review Gate:**
+After the spec review loop passes, ask the user to review the written spec before proceeding:
+
+> "Spec written and committed to `<path>`. Please review it and let me know if you want to make any changes before we start writing out the implementation plan."
+
+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.
+
 **Implementation:**
 
 - Invoke the writing-plans skill to create a detailed implementation plan