feat(lab): draft-edit lever - saved draft + Edit tool on the coding surface

Measured motivation (gen-4 acq/antitrust/tax, 2026-08-07): the exposure
echo refuses the first generate_docx and the only revision path was
re-emitting the entire deliverable - the draft is composed twice (acq:
65k vs gen-3's 41k output tokens), and a full redraft can compress
detail out of draft #1 (tax gen-4: coverage fixed at 25/25 read yet
42/77 - the misses are output-side synthesis, not read-gaps).

MIKE_DRAFT_EDIT=1 (default off, nothing changes when off - flag-off
paths return the identical tool arrays and byte-identical strings):
- A generate_docx refused by the coverage check saves its body as
  "draft.md" (LocalAssistantRequirementsState.draftMarkdown/Title/
  Filename); the refusal text teaches the revision path just-in-time.
- New Edit tool with Claude Code's exact contract (file_path/old_string/
  new_string/replace_all; unique-match or typed refusal). applyDraftEdit
  uses index splicing, never String.replace with a string pattern, so
  $-tokens in legal text stay literal (probed 7/7).
- generate_docx served with markdown optional under the flag: omitting
  it renders the saved draft as edited; re-sending a full body keeps the
  buffer in sync. Same appended-tool pattern as the requirements echo,
  so every existing arm's tool_schema_sha256 is untouched by
  construction. No system-prompt change - the gen-4 prompt sha stands.
- chat.ts flags record gains draft_edit; DRAFT_EDIT_DELTA reserved for
  the arm wiring (separate commit when the gen-4 wave lands, keeping the
  running stratum clean).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6