fix(skills): SDD review fanout scales with the change (SUP-333 #2)

subagent-driven-development mandated implementer + two-stage review +
final reviewer unconditionally — agy and opencode each dispatched 4
subagents for a one-line console.log in the 2026-06-09 quorum sweep
(cost-trivial-task-review-fanout), and the agents that passed did so
only by disobeying the skill.

- Proportionality rule: when the entire plan is one trivial,
  fully-specified mechanical change, implement directly, verify,
  commit — no review fanout. "When in doubt, it is not trivial."
  Within a multi-task plan the full pipeline still applies to every
  task regardless of size.
- Flowchart gets the trivial-exit diamond (the failing agents follow
  the flowchart literally; prose alone would not redirect them).
- Red Flags "never skip reviews" amended to reference the exception so
  the skill does not contradict itself.

TDD evidence (quorum):
- RED: agy 025324Z + opencode batches — 4 dispatches for 1 line
- GREEN: cost-trivial-task-review-fanout-opencode-20260610T002518Z-f3f5
  pass — 0 dispatches, $0.04, change landed on main checkout
- Canary: sdd-rejects-extra-features-claude-20260610T002901Z-458a pass —
  multi-task plan still runs implementer + two-stage review per task
  (tool-called Agent ✓, spec reviewer as YAGNI gate after each task)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/subagent-driven-development/SKILL.md

@@ -11,6 +11,8 @@ Execute plan by dispatching fresh subagent per task, with two-stage review after
 
 **Core principle:** Fresh subagent per task + two-stage review (spec then quality) = high quality, fast iteration
 
+**Proportionality:** Review fanout scales with the change. When the entire plan is one trivial, fully-specified mechanical change — a one-line edit, a log statement, a constant bump — implement it directly (or with a single implementer subagent), verify it, and commit. Skip the review subagents and the final reviewer: a diff with no room for interpretation has nothing for a spec or quality review to catch, and three dispatches for one line cost more than the change itself. When in doubt whether a change is trivial, it is not — run the full pipeline. Within a multi-task plan, run the full pipeline for every task regardless of size; this exception applies only when the whole plan is one trivial change.
+
 **Continuous execution:** Do not pause to check in with your human partner between tasks. Execute all tasks from the plan without stopping. The only reasons to stop are: BLOCKED status you cannot resolve, ambiguity that genuinely prevents progress, or all tasks complete. "Should I continue?" prompts and progress summaries waste their time — they asked you to execute the plan, so execute it.
 
 ## When to Use
@@ -61,11 +63,16 @@ digraph process {
     }
 
     "Read plan, extract all tasks with full text, note context, create todos" [shape=box];
+    "Entire plan = one trivial mechanical change?" [shape=diamond];
+    "Implement directly, verify, commit (no review fanout)" [shape=box];
     "More tasks remain?" [shape=diamond];
     "Dispatch final code reviewer subagent for entire implementation" [shape=box];
     "Use superpowers:finishing-a-development-branch" [shape=box style=filled fillcolor=lightgreen];
 
-    "Read plan, extract all tasks with full text, note context, create todos" -> "Dispatch implementer subagent (./implementer-prompt.md)";
+    "Read plan, extract all tasks with full text, note context, create todos" -> "Entire plan = one trivial mechanical change?";
+    "Entire plan = one trivial mechanical change?" -> "Implement directly, verify, commit (no review fanout)" [label="yes — see Proportionality"];
+    "Implement directly, verify, commit (no review fanout)" -> "Use superpowers:finishing-a-development-branch";
+    "Entire plan = one trivial mechanical change?" -> "Dispatch implementer subagent (./implementer-prompt.md)" [label="no"];
     "Dispatch implementer subagent (./implementer-prompt.md)" -> "Implementer subagent asks questions?";
     "Implementer subagent asks questions?" -> "Answer questions, provide context" [label="yes"];
     "Answer questions, provide context" -> "Dispatch implementer subagent (./implementer-prompt.md)";
@@ -237,7 +244,7 @@ Done!
 
 **Never:**
 - Start implementation on main/master branch without explicit user consent
-- Skip reviews (spec compliance OR code quality)
+- Skip reviews (spec compliance OR code quality) on a non-trivial task — the Proportionality exception covers only a plan that is one trivial mechanical change
 - Proceed with unfixed issues
 - Dispatch multiple implementation subagents in parallel (conflicts)
 - Make subagent read plan file (provide full text instead)