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>

docwriter/SKILL.md

@@ -9,10 +9,8 @@ description: Use when the API surface of one or more components has stabilized a
 
 ## Overview
 
-API documentation rots silently. Every iteration changes APIs
-and module boundaries; doc comments lag. Running this sweep
-per-cycle is waste — documenting an item that gets renamed two
-iterations later just burns context. The right moment is
+Running this sweep per-cycle is waste — documenting an item
+that gets renamed two iterations later just burns context. The right moment is
 post-stability: after a stretch of cycles in which the surface
 in question has held still. This skill is the third bucket in
 the cadence taxonomy (per-cycle-mandatory audit; orchestrator-