Add a module (plugin) system for bot features

Introduce a `Module` trait (src/bot/module.rs): a feature declares the
commands it provides and turns each invocation into a `Vec<Action>`, without
touching the connection. Modules are pure — context in, actions out — so they
are unit-testable in isolation and avoid borrowing the `Bot` that owns them.

Bot::new builds the module set and a command -> module route table; the
`commands` dispatcher routes to the owning module and executes its actions,
and `help` is auto-generated from every module's CommandSpec metadata.

The existing ping/echo/hello move into a `builtins` module, and a stateful
`dice` module (`roll [NdM]`, with its own dependency-free xorshift PRNG) is
added as a worked example. Each network gets its own module set, so stateful
modules keep independent per-network state with no locking. Adding a feature
is now a new file in bot/modules/ plus one line in modules::all().

Tests: new tests/modules.rs drives modules through the trait with no network;
end-to-end dispatch and auto-help verified against a fake server. No new deps.

src/bot/modules/builtins.rs

@@ -0,0 +1,33 @@
+//! The built-in commands: `ping`, `echo`, `hello`. (`help` is provided by the
+//! dispatcher itself, since it aggregates every module's commands.)
+
+use crate::bot::module::{Action, Command, CommandSpec, Module};
+
+/// Core commands that ship with the bot.
+pub struct Builtins;
+
+const COMMANDS: &[CommandSpec] = &[
+    CommandSpec { name: "ping", usage: "ping", about: "reply with 'pong'" },
+    CommandSpec { name: "echo", usage: "echo <text>", about: "echo the text back" },
+    CommandSpec { name: "hello", usage: "hello", about: "greet the sender" },
+];
+
+impl Module for Builtins {
+    fn name(&self) -> &'static str {
+        "builtins"
+    }
+
+    fn commands(&self) -> &'static [CommandSpec] {
+        COMMANDS
+    }
+
+    fn on_command(&mut self, cmd: &Command) -> Vec<Action> {
+        let reply = match cmd.name {
+            "ping" => "pong".to_string(),
+            "echo" => cmd.args.join(" "),
+            "hello" => format!("hello, {}!", cmd.sender),
+            _ => return Vec::new(),
+        };
+        vec![Action::Reply(reply)]
+    }
+}