nickserv: split into one file per command

Mirrors the chanserv layout: nickserv.rs keeps the service and dispatch,
each command (register, identify, logout, cert) lives in its own file
with its subcommands and parameters. Behaviour is unchanged.

modules/nickserv/register.rs

@@ -0,0 +1,17 @@
+use crate::engine::service::{Sender, ServiceCtx};
+use crate::proto::RegReply;
+
+// REGISTER <password> [email]: register the sender's current nick. The engine
+// derives the password off-thread, commits, and answers.
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx) {
+    let Some(password) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: REGISTER <password> [email]");
+        return;
+    };
+    let email = args.get(2).map(|s| s.to_string());
+    ctx.defer_register(from.nick, *password, email, RegReply::NickServ {
+        agent: me.to_string(),
+        uid: from.uid.to_string(),
+        nick: from.nick.to_string(),
+    });
+}