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/executing-plans/SKILL.md

@@ -16,7 +16,7 @@ Load plan, review critically, execute all tasks, report when complete.
 ## The Process
 
 ### Step 1: Load and Review Plan
-1. Read plan file
+1. Read plan file, and the spec it cites in its `**Spec:**` header (plans reference requirements rather than restating them)
 2. Review critically - identify any questions or concerns about the plan
 3. If concerns: Raise them with your human partner before starting
 4. If no concerns: Create todos for the plan items and proceed