Issue 49: Editor Module - Use std::expected return type

Update run_editor() to return std::expected<std::string, std::string>
for all editor-related failures instead of throwing exceptions.

Changes:
- include/editor.hpp: Change return type to std::expected
- src/editor.cpp: Replace all throw std::runtime_error() with
  return std::unexpected() for error cases:
  * Temp file open failure
  * No nvim/ set
  * Editor exit error
  * Scissors separator not found
  * Empty prompt after separator
- src/main.cpp: Use expect_or_exit() instead of try/catch

include/editor.hpp

@@ -1,5 +1,6 @@
 #pragma once
 #include "json_types.hpp"
 #include <string>
+#include <expected>
 
-std::string run_editor(HarnessState state, bool full_history);
+std::expected<std::string, std::string> run_editor(HarnessState state, bool full_history);

src/editor.cpp

@@ -52,7 +52,7 @@ public:
 };
 
 
-std::string run_editor(HarnessState state, bool full_history) {
+std::expected<std::string, std::string> run_editor(HarnessState state, bool full_history) {
     auto file_path = random_file_name();
     auto cleanup = Defer([&](){
         std::filesystem::remove(file_path);
@@ -61,7 +61,7 @@ std::string run_editor(HarnessState state, bool full_history) {
     {
         std::ofstream file(file_path);
         if (!file.is_open()) {
-            throw std::runtime_error("Failed to open temp file for editor.");
+            return std::unexpected("Failed to open temp file for editor.");
         }
 
         if (state.history.empty()) {
@@ -102,13 +102,13 @@ std::string run_editor(HarnessState state, bool full_history) {
         const char* editor_env = getenv("VISUAL");
         if (!editor_env) editor_env = getenv("EDITOR");
         if (!editor_env) {
-            throw std::runtime_error("Neither $VISUAL nor $EDITOR is set.");
+            return std::unexpected("Neither $VISUAL nor $EDITOR is set.");
         }
 
         std::string cmd = std::string(editor_env) + " " + file_path;
         int res = system(cmd.c_str());
         if (res != 0) {
-            throw std::runtime_error("Editor exited with error.");
+            return std::unexpected("Editor exited with error.");
         }
 
         std::ifstream infile(file_path);
@@ -117,7 +117,7 @@ std::string run_editor(HarnessState state, bool full_history) {
         const std::string seperator = "\n-- >8 --\n";
         size_t pos = content.find(seperator);
         if (pos == std::string::npos) {
-            throw std::runtime_error("Scissors not found.");
+            return std::unexpected("Scissors not found.");
         }
         
         size_t last_pos = std::string::npos;
@@ -130,7 +130,7 @@ std::string run_editor(HarnessState state, bool full_history) {
         extracted = trim(extracted);
         
         if (extracted.empty()) {
-            throw std::runtime_error("Prompt is empty after separator.");
+            return std::unexpected("Prompt is empty after separator.");
         }
         
         return extracted;

src/main.cpp

@@ -557,12 +557,7 @@ int main(int argc, char** argv) {
     if (prompt) {
         final_prompt = prompt;
     } else {
-        try {
-            final_prompt = run_editor(harness.serialize(), full_history);
-        } catch (const std::runtime_error& e) {
-            std::cerr << "Editor error: " << e.what() << "\n";
-            exit(1);
-        }
+        final_prompt = expect_or_exit(run_editor(harness.serialize(), full_history));
     }
 
     std::cerr << "Prompt input:" << std::endl;