refine(skills): staff-review round — fix spec-access contradiction, qualify constant bumps

Staff-review findings (4-reviewer panel):
- CONTRADICTION FIX: Spec Context said "Subagents never read the spec
  file themselves" while spec-reviewer-prompt grants exactly that
  access. Now: implementers never read it; the spec reviewer may, at
  the cited path.
- "a constant bump" was an unqualified trivial example — a one-line
  BCRYPT_ROUNDS or session-TTL change is a security-posture change;
  now qualified "with no security or behavioral consequences"
  (matching brainstorming's config-change qualifier). The diff-property
  definition adds "nothing security-relevant".
- Proportionality rewritten 146→~115 words (house style; one statement
  of the multi-task containment instead of two).
- Red Flags Never-line trimmed 33→14 words (pointer to Proportionality
  instead of third in-file restatement).
- Prompt-template rationale tails cut (the controller just read Spec
  Context; subagents need the pasted text, not the policy rationale).

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

skills/subagent-driven-development/implementer-prompt.md

@@ -12,7 +12,7 @@ Subagent (general-purpose):
 
     [FULL TEXT of task from plan - paste it here, don't make subagent read file]
 
-    [If the task cites spec sections, paste the cited sections' text here too — the plan references requirements rather than restating them]
+    [If the task cites spec sections, paste the cited sections' text here too]
 
     ## Context