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

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

- Proportionality rule: a plan that is entirely one trivial,
  fully-specified mechanical change is implemented directly, verified
  per superpowers:verification-before-completion, committed — no
  review fanout. Trivial is a property of the diff (no logic, control
  flow, or security-relevant change), not the plan's self-description;
  "a constant bump" is qualified (no security or behavioral
  consequences). Any doubt = full pipeline. Multi-task plans never
  skip reviews regardless of task size.
- Flowchart gets the matching trivial-exit diamond (the failing agents
  follow the flowchart literally).
- Red Flags "never skip reviews" points at the sole exception instead
  of contradicting it.
- writing-plans' execution handoff notes fanout scales (forward
  reference resolves within this PR's base expectations: the
  Proportionality rule ships here).

Independently mergeable: no dependency on the reference-discipline or
brainstorming-exception PRs.

Eval evidence (quorum): RED 4 dispatches for 1 line (agy, opencode);
GREEN cost-trivial-task-review-fanout opencode 3/3 pass (0 dispatches,
deterministic tool-count check) + antigravity pass (the formerly
deterministic failer); containment canary sdd-rejects-extra-features
claude 3/3 pass (full pipeline per 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 log statement, a typo fix, a constant bump with no security or behavioral consequences — implement it directly (or with a single implementer subagent), verify per superpowers:verification-before-completion (run the relevant command, confirm output), commit, and skip all review subagents, including the final reviewer: three review dispatches cost more than a one-line diff. Trivial is a property of the diff — it changes no logic, no control flow, and nothing security-relevant — not of the plan's self-description. Any doubt means not trivial: run the full pipeline. Within a multi-task plan, never skip reviews, regardless of task size.
+
 **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, fully-specified mechanical change? (any doubt = no)" [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, fully-specified mechanical change? (any doubt = no)";
+    "Entire plan = one trivial, fully-specified mechanical change? (any doubt = no)" -> "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, fully-specified mechanical change? (any doubt = no)" -> "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 — sole exception: a plan that is entirely one trivial change (see Proportionality)
 - Proceed with unfixed issues
 - Dispatch multiple implementation subagents in parallel (conflicts)
 - Make subagent read plan file (provide full text instead)

skills/writing-plans/SKILL.md

@@ -145,7 +145,7 @@ After saving the plan, offer execution choice:
 
 **If Subagent-Driven chosen:**
 - **REQUIRED SUB-SKILL:** Use superpowers:subagent-driven-development
-- Fresh subagent per task + two-stage review
+- Fresh subagent per task + two-stage review (review fanout scales with the change — see that skill's Proportionality rule)
 
 **If Inline Execution chosen:**
 - **REQUIRED SUB-SKILL:** Use superpowers:executing-plans