Fix brainstorming skill: add spec review loop to checklist and flow diagram

The spec review loop (dispatch spec-document-reviewer subagent, iterate
until approved) existed in the prose "After the Design" section but was
missing from both the checklist and the process flow diagram. Since agents
follow the diagram and checklist more reliably than prose, the spec review
step was being skipped entirely.

Added step 7 (spec review loop) to the checklist and a corresponding
"Spec review loop" → "Spec review passed?" node pair to the dot graph.

Tested with claude --plugin-dir and claude-session-driver: worker now
correctly dispatches the spec-document-reviewer subagent after writing
the design doc and before presenting to the user for review.

Fixes #677.

skills/brainstorming/SKILL.md

@@ -27,8 +27,9 @@ 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. **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
+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
 
 ## Process Flow
 
@@ -42,6 +43,8 @@ digraph brainstorming {
     "Present design sections" [shape=box];
     "User approves design?" [shape=diamond];
     "Write design doc" [shape=box];
+    "Spec review loop" [shape=box];
+    "Spec review passed?" [shape=diamond];
     "User reviews spec?" [shape=diamond];
     "Invoke writing-plans skill" [shape=doublecircle];
 
@@ -54,7 +57,10 @@ 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" -> "User reviews spec?";
+    "Write design doc" -> "Spec review loop";
+    "Spec review loop" -> "Spec review passed?";
+    "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"];
 }