feat(chat-cli): wire up logos-delivery transport and switch to client API (#91)

Replace the direct use of `conversations::Context` with `client::ChatClient`,
which is the intended public API for library consumers.

Remove `MessageEnvelope` and the username-keyed session model. The envelope
was never part of the wire protocol — sender identity was only tracked in the
CLI's local state. Chats are now keyed by conversation ID; add `/nickname` as
the user-facing replacement for named sessions.

Add a logos-delivery (Waku) transport alongside the existing file transport.
The active transport is selected at compile time: set `LOGOS_DELIVERY_LIB_DIR`
to link liblogosdelivery, otherwise the file transport is used.

Add logos-delivery as a Nix flake input and expose `.#logos-delivery` so the
library can be built with `nix build` and referenced by `LOGOS_DELIVERY_LIB_DIR`.

CI: rename `c-ffi-smoketest` to `smoketest`; add logos-delivery build step
and a `--smoketest` invocation of chat-cli to verify startup.

README.md

@@ -1,2 +1,21 @@
 # libchat
 Supporting library for Logos-chat
+
+## Example app
+
+[`bin/chat-cli`](bin/chat-cli/) is an end-to-end encrypted CLI chat app
+built on this library.  It uses [logos-delivery](https://github.com/logos-messaging/logos-delivery)
+(Waku-based) as the transport so two users anywhere in the world can chat by
+sharing an intro bundle.
+
+```sh
+# Build logos-delivery with Nix
+nix build .#logos-delivery
+# Build chat-cli with Cargo
+LOGOS_DELIVERY_LIB_DIR=./result/lib cargo build --release -p chat-cli
+# Run binary
+./target/release/chat-cli --name alice
+```
+
+See [`bin/chat-cli/README.md`](bin/chat-cli/README.md) for full build,
+run, and test instructions.