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.
