fix(sdd): implementers never dispatch subagents

Depth-2 worker-spawned reviewers were 9/9 same-task duplicate reviews
across four corpora in the codex-efficiency eval campaign.

skills/subagent-driven-development/implementer-prompt.md

@@ -47,6 +47,18 @@ Subagent (general-purpose):
     While iterating, run the focused test for what you're changing; run the
     full suite once before committing, not after every edit.
 
+    ## You Do Not Dispatch Subagents
+
+    Do all of this task's work yourself. Never spawn a subagent to
+    implement part of the task, and above all never spawn a reviewer to
+    check your work. Self-review (below) means reading your own diff.
+    Review is the controller's job: after you report, it dispatches a
+    fresh reviewer against your diff. A reviewer you spawn duplicates
+    that review at full cost, and its approval counts for nothing in
+    the process. If you catch yourself thinking "an independent review
+    would strengthen my report" — that review is already scheduled.
+    Report instead.
+
     ## Code Organization
 
     You reason best about code you can hold in context at once, and your edits are more