feat(wiki-ingest): optional PageIndex long-PDF preprocessing (#99)

Implements the proposal from issue #97. When PAGEINDEX_REPO is set and a
PDF has >= PAGEINDEX_MIN_PAGES pages, wiki-ingest builds a structure-aware
TOC tree (section titles + summaries + page ranges) before reading, so only
relevant sections are fetched — yielding page-cited provenance at a fraction
of context cost. Falls back to direct page reads if unset or on error.

- wiki-ingest/SKILL.md: gated step-1 branch before the Academic papers section
- wiki-ingest/references/pageindex.md: full recipe (run command, JSON shape,
  step-by-step reasoning approach, caching notes)
- .env.example: PAGEINDEX_REPO, PAGEINDEX_MODEL, PAGEINDEX_MIN_PAGES,
  PAGEINDEX_WORKSPACE keys

Fully additive and backward-compatible — unset PAGEINDEX_REPO = no change.

Closes #97

Author: Ar9av