fix(skills): description-level exceptions are authoritative in the routing rule (SUP-333 #4)

Adversarial review findings D1/D2: the 1%-chance invocation rule and
the "Add X doesn't mean skip workflows" line contradicted the new
brainstorming description exception in both directions — a compliant
agent re-imposes the cost failure (invocation itself is the measured
cost event), while a cost-optimizing agent could treat any skip as
sanctioned. The routing skill now states: a documented exception in a
skill's own description defines that skill's scope (compliance, not
rationalization); any doubt about the exception's conditions means
invoke; and only the description can define one — agents cannot infer
exceptions.

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

skills/using-superpowers/SKILL.md

@@ -49,6 +49,8 @@ Skills speak in actions ("dispatch a subagent", "create a todo", "read a file")
 
 **Invoke relevant or requested skills BEFORE any response or action.** Even a 1% chance a skill might apply means that you should invoke the skill to check. If an invoked skill turns out to be wrong for the situation, you don't need to use it.
 
+**Documented exceptions in a skill's own description are authoritative.** When a description itself says the skill does not apply to a request (e.g. brainstorming's nothing-to-design exception), not invoking it is compliance, not rationalization — the description defines the skill's scope. Any doubt about whether the exception's conditions hold means invoke. Only the skill's description can define such an exception; you cannot infer one.
+
 ```dot
 digraph skill_flow {
     "User message received" [shape=doublecircle];
@@ -118,4 +120,4 @@ The skill itself tells you which.
 
 ## User Instructions
 
-Instructions say WHAT, not HOW. "Add X" or "Fix Y" doesn't mean skip workflows.
+Instructions say WHAT, not HOW. "Add X" or "Fix Y" doesn't mean skip workflows — unless a skill's own description exempts the request (see The Rule above).