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>

planner/SKILL.md

@@ -72,13 +72,9 @@ Read the returned file-map. Lift the relevant entries into
 the plan's "Files this plan creates or modifies" section; the
 orchestrator may add or amend entries that recon missed.
 
-Dispatch carrier — the fields `spec_path`, `iteration_scope`, and
-`focus_hint`, defined authoritatively under **Carrier contract**
-in `agents/plan-recon.md` (which marks `spec_path`/`iteration_scope`
-mandatory and `focus_hint` optional, and specifies the
-BLOCKED-on-missing rule). That table is the single source for those
-fields; it is deliberately not restated here, so the two files
-cannot drift.
+Dispatch carrier: see the **Carrier contract** table in
+`agents/plan-recon.md` (single source for `spec_path`,
+`iteration_scope`, `focus_hint`).
 
 Recon does NOT decide decomposition. Two recon dispatches per
 planner run is unusual but legitimate (e.g. one for the main