fix(skills): brainstorming gate exempts nothing-to-design requests; description exceptions are authoritative (SUP-333 C)

Consolidates the brainstorming exception with its routing-layer
semantics, so this PR is independently mergeable (previously split
across two stacked PRs whose intermediate state left the always-
injected routing text contradicting the shipped description).

brainstorming: the nothing-to-design exception, earned by a tripwire
scan stated in one line before acting. Tripwires precede the
permission (skimmers stop at "implement directly"); security-posture
touches re-gate even with the exact value stated; requested deletions
re-gate; rationalization table per writing-skills bulletproofing.
Description 971/1024 chars, YAML-validated.

using-superpowers: description-level exceptions are authoritative
(compliance, not rationalization); doubt means invoke; only the
description can define one; the skip must state its scan; flowchart
routes the exempt path through the scan statement;
<EXTREMELY-IMPORTANT> defers in one parenthetical.

writing-skills: negative triggering conditions are scope (allowed,
required at the description) vs workflow summaries (still forbidden) —
prevents a future checklist pass from stripping the exception.

Eval evidence (quorum): RED cost-checkbox-over-trigger failed 5/6
agents (pi ⊘); GREEN claude 3/3, codex ✓, antigravity ✓ (kimi
unchanged from baseline — does not read description exceptions);
gate-still-fires: brainstorming-resists 2/2 + codex, spec-plan
brainstorm leg 3/3. Boundary scenarios (security one-liner, requested
deletion): pre-stack dev baseline 0/3 + 0/3 (silent edit every time —
the blanket gate never fired on one-liners); this text 2/3 + 2/3, the
first text in the corpus to catch these at any rate; scenarios ship as
regression instruments (proposed in prime-radiant-inc/superpowers-evals#11, open).

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

Canary-caught addition: on the assembled text, triggering-writing-plans
went 0/3 with claude citing "your explicit instruction wins per the
priority rules" to skip writing-plans under the scenario's "don't ask
me any questions" pressure — the Instruction Priority section read as
licensing ad-hoc pressure to skip workflow steps. User Instructions now
distinguishes pressure phrasing (changes interaction style) from
instructions that name what to skip (honored), and tags the quoted
rationalization.

skills/writing-skills/SKILL.md

@@ -151,6 +151,8 @@ Concrete results
 
 The description should ONLY describe triggering conditions. Do NOT summarize the skill's process or workflow in the description.
 
+(Negative triggering conditions are still triggering conditions: a description MAY state when the skill does NOT apply — including its tripwires — and per using-superpowers' Rule such description-level exceptions are authoritative, so they must live here, not only in the body. That is scope, not workflow.)
+
 **Why this matters:** Testing revealed that when a description summarizes the skill's workflow, an agent may follow the description instead of reading the full skill content. A description saying "code review between tasks" caused an agent to do ONE review, even though the skill's flowchart clearly showed TWO reviews (spec compliance then code quality).
 
 When the description was changed to just "Use when executing implementation plans with independent tasks" (no workflow summary), the agent correctly read the flowchart and followed the two-stage review process.