P2 slice 7: backups and restore (D-30, D-31)

The server backs each book up itself: a per-book git archive, pushed
fast-forward only to a git remote and to a folder, when the books open,
after a quiet spell, before they close and on request; every commit,
push and drill is a record in ops/backup.jsonl. Drills clone each
target and verify the chain, the blobs, the vault and the head, weekly
and after a failure. `wanderbooks books restore` previews, then
restores beside or (with --replace) over, never moving books something
holds. The archive refuses anything but a plain bare repository and
pins git against the owner's configuration. books.backup and
books.back_up_now for the agent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-mcp/src/handbook.rs

@@ -81,6 +81,7 @@ pub const SECTIONS: &[(&str, &str)] = &[
         include_str!("../handbook/settings-and-conventions.md"),
     ),
     ("viewer", include_str!("../handbook/viewer.md")),
+    ("backups", include_str!("../handbook/backups.md")),
     (
         "rules-that-change",
         include_str!("../handbook/rules-that-change.md"),