cli: sift extract uploads a local PDF; README and skill

`sift extract <file.pdf>` stats the file (refusing over 100 MiB before the
upload), streams it as the POST body with Content-Type application/pdf and the
ContentLength set, and prints the /v1/extract answer exactly as `sift fetch`
prints a PDF URL — same content output, same errors and exitFor codes.
parseExtractOpts reuses splitAroundPositional; extractURL sends only the set
options with mode=digest when a question is given. main.go gains the case and a
help line; README and skill document the command.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

skill/SKILL.md

@@ -60,6 +60,9 @@ for blocked pages), `--json` (full envelope: content, source, context, sizes, wa
 PDFs work: `sift fetch` extracts them and returns the text. A short one comes back whole; a
 long one comes back as a map — an outline with page numbers, then the first pages. Then:
 
+- A PDF on disk (an attachment, a download): `sift extract <file.pdf>` uploads it to `/v1/extract`
+  and prints the answer as `sift fetch` does, with the same `--pages`/`--grep`/`--question`.
+  Never run `pdftotext` yourself.
 - `sift fetch URL --pages 41-60` for a page range, `--grep 'pattern'` for matching lines, or
   `--question "…"` for a digest that cites each fact as `(p. N)`; `--question` works on long
   PDFs, and the first answer may be `bm25` (lexical) — ask again later for a sharper one.