feat(lab): silo'd derived-section-index arm (mike_markdown_e2e_index_v1)

Consumers-only experiment on the markdown-e2e surface: derive the .docx
section tree at ingest with existing detectors (extractDocxBodyStructure ->
compileAgreementSkeleton) and prepend a compact SECT-INDEX to the served
markdown so the model can orient and read selectively instead of whole-reading.

Design (measured): the markdown already states top-level section numbers in
its headings and pandoc does not flatten subsections ((a) items are literal
docx text); the only identity the markdown lacks is the COMPOSED subsection
label (Section 2.01(a)), which the skeleton derives. Inline markers would
duplicate headings and add tokens without reducing reads; the index carries
the derived tree once (~5.2k tokens on a 146k-char credit agreement, 310
sections) and is the only form that enables targeted reads.

Silo: one flag (MIKE_STRUCTURE_INDEX), one new module (structureIndexExperiment),
one gated branch in the docx read path, one prompt const + delta, one arm entry.
No detector or write-grammar changes; markdown body byte-identical off-arm.
localAssistantTools.ts staged hunks only (concurrent citator session edits in
that file left unstaged).

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