feat(lab): coding_markdown_v1 - pure-coding toolset over the pandoc-markdown plane

Eli 2026-08-06: assume the coding-agent toolset is strongest if built
right; don't reinvent the wheel. The arm is the frozen lean-batch chassis
(list_documents/Grep/Read/generate_docx - the tool shapes the models were
RL'd on, executors reused verbatim) with exactly two attributable deltas:

1. MIKE_READ_DOCX_MARKDOWN now reaches the coding executors: Glob/Read/
   Grep list, search, and read the pandoc markdown drafting plane (the
   proven token lever), via codingDocument() sharing the per-turn
   drafting cache. file:line coordinates agree arm-wide; slices start
   past any attached SECT-INDEX; non-docx keeps plaintext. No frozen arm
   combines this flag with a coding shape, so the reach is inert there
   (conformance now asserts it per arm).
2. MIKE_CODING_NEUTRAL_PROMPT: the SOURCE WORK navigation prescriptions
   are REMOVED (no read-once, no batch-vs-grep guidance) - run 1 observes
   the model's native pathway choices. Write-side discipline stays:
   grounding/clean-reads, one completeness check, terminal authoring.

Motivation: the v2 exposure-forcing null (acq 26/64, tax 42/77; pooled ~0
vs v1 at +30-50% input) - breadth forcing reshuffles criteria under a
fixed synthesis budget; the live lever is evidence-driven selection.

Proof: plane probe both states (Glob markdown vs plaintext plane; grep
hit -> Read round-trip at the same file:line; hit text verbatim in
pandoc markdown); served-surface probe coding mode 4/4 (sha, flags,
no-prescription assert) + treatment-mode regression 9/9; tsc clean.

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