cli: new books dirs get a SessionStart hook that checks the viewer

The viewer offer (open_at_start) lived only in AGENTS.md, so it depended
on the agent remembering it on its first turn. The scaffold's
.claude/settings.json now runs just gui-status at session start and hands
the agent the result plus open_at_start.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tpv1gBaepKjQiAVJqejGbA

crates/ledger-cli/tests/init.rs

@@ -97,6 +97,11 @@ fn the_server_is_registered_for_this_dir() {
         settings["enabledMcpjsonServers"],
         serde_json::json!(["wanderbooks"])
     );
+    // Every session starts by checking the viewer against open_at_start.
+    let hook = settings["hooks"]["SessionStart"][0]["hooks"][0]["command"]
+        .as_str()
+        .expect("SessionStart hook command");
+    assert!(hook.contains("just gui-status") && hook.contains("open_at_start"));
     // The justfile opens this dir's books in the GUI.
     let just = fs::read_to_string(books.join("justfile")).unwrap();
     assert!(just.contains("ledger-gui\" --data-dir \"{{justfile_directory()}}/books-data\""));