refine(skills): staff-review round — trim reference rule, close executing-plans spec gap

Staff-review findings (4-reviewer panel):
- Reference paragraph rewritten 170→123 words preserving every
  behavioral condition (paraphrase/summarize coverage, no-skip guard,
  WHAT-WHY/HOW split, No Placeholders boundary, drift counter,
  zero-context rescope); fixes the "(brainstorming did)" syntax.
- **Spec:** header bracket: cut the never-skip sermon duplicated from
  the Overview (same loaded document); the conditional none-branch
  stays.
- executing-plans Step 1 now reads the spec the plan cites — plans are
  no longer self-contained, and the non-subagent execution path was
  never told (the eval only exercised the SDD consumer).
- writing-plans plan-location preference line gets the same
  existing-dir-is-not-a-preference guard as the spec path.
- brainstorming: deduplicate the docs/specs/ prohibition (step 6
  parenthetical stays; After-the-Design bullet was the second
  statement in one file).

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

skills/brainstorming/SKILL.md

@@ -109,7 +109,6 @@ digraph brainstorming {
 **Documentation:**
 
 - Write the validated design (spec) to `docs/superpowers/specs/YYYY-MM-DD-<topic>-design.md`
-  - The `docs/superpowers/` prefix is the convention; do not shorten it to `docs/specs/`
   - (An explicit user instruction overrides this default; an existing differently-named docs directory does not)
 - Use elements-of-style:writing-clearly-and-concisely skill if available
 - Commit the design document to git