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>

implement/SKILL.md

@@ -243,8 +243,7 @@ independently at cycle close.
   does not permit a subagent to spawn other subagents. The
   orchestrator-agent cannot dispatch the role-agents above;
   it adopts each role as a sequential phase in its own
-  context. This was a known platform constraint at the time
-  the role was designed.
+  context.
 - **Input sources:**
   - `../planner/SKILL.md` — produces the plan files this
     skill consumes