refactor: remove client-ffi and legacy nim bindings (#133)

closes: #77

The C consumer story lives downstream now: logos-chat-module wraps the
client crate and exposes its own C API. The in-tree client-ffi crate has
no consumers left, and the nim bindings still target the removed
Context-based C API.

- delete crates/client-ffi (including the message-exchange C example)
  and nim-bindings
- drop core/conversations' unused safer-ffi dependency plus the leftover
  C artifact crate-types: staticlib on core/conversations, cdylib on
  double-ratchets (neither crate has extern "C" exports)
- flake.nix: drop the default package (it built libclient_ffi.a plus its
  header); keep the logos-delivery package and the dev shell
- ci.yml: drop the C FFI smoketest steps (valgrind included), the rustup
  install the smoketest no longer needs, and the nix-build job that
  built the removed default package
- ADR 0001: point the FFI-compatibility driver at the downstream C API
  boundary instead of crates/client-ffi

.gitignore

@@ -22,11 +22,6 @@ target
 
 */.DS_Store
 
-# Compiled binary
-**/ffi_nim_example
-/nim-bindings/examples/pingpong
-/nim-bindings/libchat
-
 # Temporary data folder
 tmp
 
@@ -34,9 +29,3 @@ tmp
 result
 
 .DS_Store
-
-# Generated C headers (produced by `make` in examples/c-ffi; do not commit)
-crates/client-ffi/client_ffi.h
-
-# Compiled C FFI example binary
-crates/client-ffi/examples/message-exchange/c-client