fix(lab): restore multi-deliverable authoring in coding arms

The create contract ("a successful call ends the turn", "Create the
requested Word deliverable once") taught single-and-done, which made the
4-DOCX prepare-antitrust task structurally fail the deliverable gate
(authored 1/4). The enforcement layer was never the cap: chat.ts
terminalCreateBatch marks a whole round of create-calls terminal when
every call commits action:created, so N generate_docx calls in one final
response already yield N deliverables. This is a capability fix, not a
treatment arm - levers accrete in place per 2026-08-06.

- CODING_MARKDOWN_LAB_SYSTEM_PROMPT + LEAN_BATCH_LAB_SYSTEM_PROMPT:
  "emit one generate_docx call per requested deliverable - a single
  response may carry several calls - and the turn ends after the batch
  is committed"
- DOMAIN_PROMPTS.output_document: drop "once" -> "each requested
  Word deliverable"
- refine note (draft-edit-v4): single-deliverable requests render the
  saved draft without re-sending; multi-deliverable requests must
  re-send each complete body (the draft buffer holds one document -
  without this, all N docs would clone the last body)
- receipt delta multi_deliverable (multi-deliverable-authoring-v1) on
  the three coding arms; conformance re-derives the expected prompt sha
  from the same constants so the prompt-hash gate stays honest

Co-Authored-By: Claude <noreply@anthropic.com>