Documentation site: mdBook user + contributor guide (2.2)

An mdBook under docs/ rendering eight pages: introduction,
installation, first-run/config, downloading, an anti-bot guide that
captures the hard-won cookies/curl_cffi/POT/player-client knowledge,
troubleshooting (the nine error classes + non-download issues),
architecture (the two-front-ends/one-engine design, settings flow,
layout invariant, persistence), and packaging.

- .forgejo/workflows/docs.yml builds the book and publishes docs/book/
  to the `pages` branch for Codeberg Pages on docs/ changes.
- docs/book/ is gitignored (rendered output).
- README: fix the stale backup.directory comment — every platform nests
  under the one library root, not a channels/ sibling split.
- Add CLAUDE.md (repo guidance for Claude Code).

Builds clean with mdbook 0.5; intra-doc anchor links verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/docs.yml

@@ -0,0 +1,44 @@
+# Build the mdBook docs and publish them to Codeberg Pages.
+#
+# Codeberg serves the `pages` branch of a repo (or a dedicated `pages`
+# repo) at https://<user>.codeberg.page/<repo>/. This workflow builds the
+# book on a push to main that touches docs/ and pushes the rendered HTML
+# to the `pages` branch.
+name: docs
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'docs/**'
+      - '.forgejo/workflows/docs.yml'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: rust:1.85-bookworm
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install mdBook
+        run: cargo install mdbook --no-default-features --features search
+
+      - name: Build the book
+        run: mdbook build docs
+
+      - name: Publish to the pages branch
+        # forgejo-release isn't right here; push the built docs/book/ to a
+        # `pages` branch that Codeberg Pages serves. Uses the repo token.
+        env:
+          TOKEN: ${{ secrets.PAGES_TOKEN }}
+        run: |
+          cd docs/book
+          git init -q
+          git config user.name "ci"
+          git config user.email "ci@localhost"
+          git add -A
+          git commit -q -m "docs build ${{ github.sha }}"
+          git push -f "https://x:${TOKEN}@codeberg.org/${{ github.repository }}.git" HEAD:pages