Add paper CLI: download academic papers by DOI and convert to markdown

Downloads PDFs from LibGen (primary) or Anna's Archive API (fallback),
converts to markdown via marker_single, and prints to stdout. Includes
XDG-compliant caching, nix flake with marker-pdf packaging, and a
Claude Code skill for paper-reader integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-19-paper-cli-design.md

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+# paper CLI — Design
+
+A CLI tool that downloads academic papers by DOI from Anna's Archive and converts them to markdown.
+
+## CLI Interface
+
+```
+paper <DOI>
+```
+
+Single positional argument. Markdown output goes to stdout.
+
+```
+paper 10.1038/nature12373 > paper.md
+```
+
+## Download Flow
+
+1. Request `https://annas-archive.org/scidb/<DOI>` with a browser-like User-Agent
+2. Parse HTML for `<iframe>` or `<embed>` with `id="pdf"` — extract `src` for direct PDF URL
+3. Fallback: find any link ending in `.pdf`
+4. Download PDF to a temp file
+5. Exit with clear error if no PDF found
+
+## Conversion
+
+1. Shell out to `marker_single <tempfile.pdf> --output_dir <tempdir>`
+2. Read the generated `.md` file from the output dir
+3. Print to stdout
+4. Clean up temp dir
+
+## Error Handling
+
+- `marker_single` not on PATH: tell user to install (`pip install marker-pdf`)
+- Conversion failure: forward marker's stderr
+- Network errors: surface reqwest errors clearly
+- No PDF found: specific error message with the DOI
+
+## Dependencies
+
+- `clap` — argument parsing
+- `reqwest` (blocking, rustls-tls) — HTTP
+- `scraper` — HTML parsing
+- `tempfile` — temp directory
+- `anyhow` — error handling
+
+## Dev Environment
+
+The nix flake includes Rust nightly toolchain and marker-pdf in the devshell.