feat: write runtime port file for frontend discovery

After binding to a port (which may differ from configured due to auto-retry),
writes the actual port number to {config_dir}/port. Removes the file on
shutdown to prevent stale discovery.

Frontends can read ~/Library/Application Support/{PluginName}/port to find
the correct port, then verify via /health that the plugin identity matches.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ByteBard97

plugin/src/HttpServer.cpp

@@ -27,6 +27,8 @@
 #include <nlohmann/json.hpp>
 
 #include <chrono>
+#include <cstdio>
+#include <fstream>
 #include <memory>
 
 using json = nlohmann::json;
@@ -143,6 +145,12 @@ void HttpServer::Stop() {
     gServer.reset();
   }
 
+  // Remove runtime port file so stale ports aren't discovered
+  {
+    std::string portFilePath = ConfigManager::GetConfigDir() + "/port";
+    std::remove(portFilePath.c_str());
+  }
+
   ready_.store(false);
 }
 
@@ -602,6 +610,17 @@ void HttpServer::ServerThread() {
     }
   }
 
+  // Write a runtime port file so frontends can discover which port we bound to.
+  // This is especially useful when port auto-retry shifted us to a different port.
+  // File goes next to config: ~/Library/Application Support/{PluginName}/port
+  {
+    std::string portFilePath = ConfigManager::GetConfigDir() + "/port";
+    std::ofstream portFile(portFilePath);
+    if (portFile.is_open()) {
+      portFile << port_;
+    }
+  }
+
   // Mark as ready
   ready_.store(true);