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>

audit/SKILL.md

@@ -25,9 +25,7 @@ explicit backlog issue (issue tracker configured under
 - the reason for deferral,
 - the date the audit will be re-run.
 
-"We want to keep moving" is not a valid reason. Conventions
-require deferred audits to compound, which is the failure mode
-the skill prevents.
+"We want to keep moving" is not a valid reason.
 
 Also triggered manually when baseline drift is suspected
 outside a cycle close (e.g. the bencher reports an unexplained