feat: implement multi-process LSP server architecture (#364)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/feature/code_completion.cpp

@@ -24,8 +24,6 @@ namespace clice::feature {
 
 namespace {
 
-namespace protocol = eventide::language::protocol;
-
 struct CompletionPrefix {
     LocalSourceRange range;
     llvm::StringRef spelling;