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>

fieldtest/SKILL.md

@@ -115,17 +115,14 @@ carrier.
 
 Dispatch `fieldtester` with the carrier from the Handoff
 Contract below. The agent picks 2-4 examples (one per axis
-the cycle touched), implements them as downstream consumers,
-runs them, classifies findings, and writes the spec. All
-artefacts (fixtures + spec) stay in the working tree as
-unstaged changes; the orchestrator commits them after
+the cycle touched; fewer loses the signal on variation, more
+overflows one readable report), implements them as downstream
+consumers, runs them, classifies findings, and writes the
+spec. All artefacts (fixtures + spec) stay in the working tree
+as unstaged changes; the orchestrator commits them after
 reviewing the report (suggested commit subject:
 `fieldtest: <cycle> — <N> examples, <K> findings`).
 
-Two to four examples is the right size. One is too few (no
-signal on variation); five is too many for one report to
-stay readable.
-
 ## Handoff Contract
 
 `fieldtest` consumes (from orchestrator at cycle close) the carrier