skills: cut blabla and redundancy from skill prose

Per-skill prose tightening from the same audit swarm; each finding
passed an adversarial second reviewer. No behavioural change.

- audit: drop garbled "Conventions require deferred audits to
  compound" sentence (says the opposite of intent; the preceding
  line already closes the loophole).
- brainstorm: drop forward-pointing meta-comment about the
  Rationalisations table.
- debug: drop third restatement that debugger.md is the single
  source for the carrier/handoff fields.
- docwriter: drop motivational opener; Overview starts at the
  waste argument.
- fieldtest: fold the 2-4-examples rationale into the dispatch
  sentence instead of restating the count a fourth time.
- implement: drop "known platform constraint at the time" aside.
- planner: cut the verbose anti-drift paragraph (which restated
  the very table it claimed not to) down to a cross-reference.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

debug/SKILL.md

@@ -87,11 +87,8 @@ separate one.
 
 - **Agent dispatched:** `agents/debugger.md` — carries the
   four-phase process, the Phase 4.5 escalation rule, the
-  Common Rationalisations table, and the Red Flags list. It is
-  also the single source for the carrier and handoff field
-  definitions (Carrier contract / Output format); this skill
-  names the fields but does not redefine them. The orchestrator
-  does not execute these phases directly.
+  Common Rationalisations table, and the Red Flags list. The
+  orchestrator does not execute these phases directly.
 - **Hand-off target:** `../implement/SKILL.md` — runs the
   GREEN side after the orchestrator has decided whether to
   commit the RED test separately or as part of the final fix