P5b and P5c plans: long-document retrieval (BM25 first, background embedding) and /v1/extract + sift extract; given tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/p5c/README.md

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+# P5c implementation plan: PDFs already on a host
+
+> **For the implementing model:** do not work from this file. The owner gives you one task file
+> at a time. This file is the index for the owner and the reviewer.
+
+**Goal:** an agent with a PDF on disk (a Mattermost attachment, a download) gets the same
+answer as for a PDF URL — text with page markers, the map, pages, grep, cited digests — by
+uploading the bytes: `POST /v1/extract`, CLI `sift extract <file>`. No poppler on the host, no
+shell `pdftotext`. The cache key is the content hash, so the same file is extracted once.
+
+| Task | Package | Given |
+|---|---|---|
+| 01-extract-api | `internal/api` | `api/extract_test.go` |
+| 02-extract-cli | `cmd/sift`, `README.md`, `skill/SKILL.md` | `cmd/sift/extract_test.go` |
+
+Runs after P5b (task 01 uses the P5b digest fields). Given tests compiled against a panic-only
+skeleton on master `1311cf3` and failed for the intended reasons.
+
+## Changes during the run
+
+(none yet)