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.

bin/chat-cli/Cargo.toml

@@ -8,11 +8,19 @@ name = "chat-cli"
 path = "src/main.rs"
 
 [dependencies]
-libchat = { path = "../../core/conversations" }
+client = { path = "../../crates/client" }
+
 ratatui = "0.29"
 crossterm = "0.29"
+clap = { version = "4", features = ["derive"] }
 anyhow = "1.0"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-hex = "0.4"
 arboard = "3"
+base64 = "0.22"
+thiserror = "2"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(logos_delivery)'] }