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.

flake.lock

@@ -1,6 +1,42 @@
 {
   "nodes": {
+    "logos-delivery": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay",
+        "zerokit": "zerokit"
+      },
+      "locked": {
+        "lastModified": 1777287099,
+        "narHash": "sha256-H2gpbDUg6Wy+uIY9wL0t9ICUPN82B/vCnXZ2mo3Wa/E=",
+        "owner": "logos-messaging",
+        "repo": "logos-delivery",
+        "rev": "5034086fefe2f32bf95319cdd39aa62fc622e4bc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "logos-messaging",
+        "repo": "logos-delivery",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
+      "locked": {
+        "lastModified": 1770464364,
+        "narHash": "sha256-z5NJPSBwsLf/OfD8WTmh79tlSU8XgIbwmk6qB1/TFzY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "23d72dabcb3b12469f57b37170fcbc1789bd7457",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "23d72dabcb3b12469f57b37170fcbc1789bd7457",
+        "type": "github"
+      }
+    },
+    "nixpkgs_2": {
       "locked": {
         "lastModified": 1775710090,
         "narHash": "sha256-ar3rofg+awPB8QXDaFJhJ2jJhu+KqN/PRCXeyuXR76E=",
@@ -18,11 +54,55 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs",
-        "rust-overlay": "rust-overlay"
+        "logos-delivery": "logos-delivery",
+        "nixpkgs": "nixpkgs_2",
+        "rust-overlay": "rust-overlay_3"
       }
     },
     "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "logos-delivery",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1775099554,
+        "narHash": "sha256-3xBsGnGDLOFtnPZ1D3j2LU19wpAlYefRKTlkv648rU0=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "8d6387ed6d8e6e6672fd3ed4b61b59d44b124d99",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "rust-overlay_2": {
+      "inputs": {
+        "nixpkgs": [
+          "logos-delivery",
+          "zerokit",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1771211437,
+        "narHash": "sha256-lcNK438i4DGtyA+bPXXyVLHVmJjYpVKmpux9WASa3ro=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "c62195b3d6e1bb11e0c2fb2a494117d3b55d410f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "rust-overlay_3": {
       "inputs": {
         "nixpkgs": [
           "nixpkgs"
@@ -41,6 +121,29 @@
         "repo": "rust-overlay",
         "type": "github"
       }
+    },
+    "zerokit": {
+      "inputs": {
+        "nixpkgs": [
+          "logos-delivery",
+          "nixpkgs"
+        ],
+        "rust-overlay": "rust-overlay_2"
+      },
+      "locked": {
+        "lastModified": 1771279884,
+        "narHash": "sha256-tzkQPwSl4vPTUo1ixHh6NCENjsBDroMKTjifg2q8QX8=",
+        "owner": "vacp2p",
+        "repo": "zerokit",
+        "rev": "53b18098e6d5d046e3eb1ac338a8f4f651432477",
+        "type": "github"
+      },
+      "original": {
+        "owner": "vacp2p",
+        "repo": "zerokit",
+        "rev": "53b18098e6d5d046e3eb1ac338a8f4f651432477",
+        "type": "github"
+      }
     }
   },
   "root": "root",