ANW-27 CLI: anwesen merge -- one-shot local markdown-merge to stdout

Thin local front-end over ANW-26's merge engine (query::execute_merge):
walk the vault once (vault::scan, same as doctor), evaluate the --query
string, assemble the merged document, write it to stdout. No HTTP, no
watcher, no index. Byte-identical to the HTTP merge mode for the same
vault and query.

src/lib.rs

@@ -10,6 +10,7 @@ pub mod app;
 pub mod doctor;
 pub mod health;
 pub mod http;
+pub mod merge;
 pub mod query;
 pub mod store;
 pub mod vault;