paper-fetch

# Paper Fetch If the caller already has the paper — a local `.md`/`.txt`/`.pdf` path, or a direct HTTP(S) PDF URL (path ends in `.pdf`, ignoring query and fragment) — read it directly before considering any search route. Record `source_channel` as `local_file`, `local_pdf`, or `direct_pdf`. If the read fails, return `not_found` and do not fall back to alphaxiv, Semantic Scholar, bioRxiv, or medRxiv. The pipeline's sole entry point: retrieves a paper and lands it on disk. It checks the cache first, reads already-identified sources (local files, direct PDF URLs) with no search at all, and otherwise uses the fixed fallback (alphaxiv → Semantic Scholar routing → bioRxiv/medRxiv → not_found). Decoupled from `literature-engine`'s `literature-research`/`literature-search`/ `literature-overview` — this SOP holds its own retrieval calls rather than delegating. ## Why an already-supplied file still enters through this SOP Skipping straight to a reading SOP with the caller's own path looks like it saves a step, but 13 downstream SOPs take `meta_path` and read only section line ranges (`star-awarding` reads method + results; `first-pass-skim`'s "headings only, never bodies" constraint holds *because* it is handed shallow ranges). A bare path carries no index, and a bare `.pdf` carries no extracted text at all. So the thing to skip is the **four-channel search**, not the landing and indexing. That is what Step 1 does: no network lookup, same landing step, same output contract. Tactic...