feat: merge MCP config when installing bundled mcp config

src/config/mod.rs

@@ -445,9 +445,10 @@ fn confirm_asset_overwrite(category: AssetCategory, label: &str, target: &Path)
     }
     let body = match category {
         AssetCategory::McpConfig => format!(
-            "This replaces your MCP server configuration at {} with this \
-             build's bundled template. Your configured MCP servers (and any \
-             custom secret references they contain) will be lost.",
+            "This merges the bundled MCP server template into your configuration \
+             at {}. New servers from the bundled template will be added; any \
+             MCP servers you have already configured (including custom secret \
+             references) are left untouched.",
             target.display()
         ),
         _ => format!(

src/config/request_context.rs

@@ -5175,19 +5175,26 @@ mod tests {
 
     #[test]
     #[serial]
-    fn install_mcp_config_overwrites_existing() {
+    fn install_mcp_config_merges_existing() {
         let _guard = TestConfigDirGuard::new();
 
         Functions::install_mcp_config().unwrap();
         let mcp = paths::mcp_config_file();
         assert!(mcp.exists(), "install_mcp_config should create mcp.json");
 
-        write(&mcp, "USER_MCP_CONFIG").unwrap();
+        let custom_json =
+            r#"{"mcpServers":{"my-custom-server":{"type":"stdio","command":"custom-cmd"}}}"#;
+        write(&mcp, custom_json).unwrap();
         Functions::install_mcp_config().unwrap();
-        assert_ne!(
-            read_to_string(&mcp).unwrap(),
-            "USER_MCP_CONFIG",
-            "install_mcp_config must overwrite the existing mcp.json"
+
+        let result = read_to_string(&mcp).unwrap();
+        assert!(
+            result.contains("my-custom-server"),
+            "install_mcp_config must preserve user-added MCP servers"
+        );
+        assert!(
+            result.contains("github"),
+            "install_mcp_config must add new bundled servers"
         );
     }
 }