feat(nvim-core): implement nvim_set_extmark for rich virtual text UI feedback

- Added nvim_set_extmark tool schema and handler
- Implemented lua bridge using vim.api.nvim_buf_set_extmark to dynamically overlay ghost text and highlight groups onto active buffers

nvim-core/src/lib.rs

@@ -510,6 +510,31 @@ async fn set_nvim_diagnostics(line: i64, message: &str) -> Result<(), String> {
     Ok(())
 }
 
+async fn set_nvim_extmark(line: i64, text: &str, hl_group: &str) -> Result<(), String> {
+    let escaped_text = text.replace('\\', "\\\\").replace('"', "\\\"");
+    let lua_script = format!(
+        r#"
+        local ns = vim.api.nvim_create_namespace("gemini_extmarks")
+        local opts = {{
+            virt_text = {{{{"{escaped_text}", "{hl_group}"}}}},
+            virt_text_pos = "eol",
+        }}
+        vim.api.nvim_buf_set_extmark(0, ns, {line} - 1, 0, opts)
+    "#
+    );
+
+    call_nvim_method(
+        "nvim_exec_lua",
+        vec![
+            rmpv::Value::String(lua_script.into()),
+            rmpv::Value::Array(vec![]),
+        ],
+    )
+    .await?;
+
+    Ok(())
+}
+
 fn rmpv_to_json(val: &rmpv::Value) -> serde_json::Value {
     match val {
         rmpv::Value::Nil => serde_json::Value::Null,
@@ -731,6 +756,19 @@ pub async fn run_mcp_loop(app_name: &str, app_version: &str) {
                                         "required": ["line", "message"]
                                     }
                                 },
+                                {
+                                    "name": "nvim_set_extmark",
+                                    "description": "Add virtual text (ghost text) to a specific line in the buffer to give the user rich UI feedback",
+                                    "inputSchema": {
+                                        "type": "object",
+                                        "properties": {
+                                            "line": { "type": "integer" },
+                                            "text": { "type": "string" },
+                                            "highlight_group": { "type": "string", "description": "Optional. E.g., 'DiagnosticVirtualTextInfo', 'Comment'" }
+                                        },
+                                        "required": ["line", "text"]
+                                    }
+                                },
                                 {
                                     "name": "nvim_list_buffers",
                                     "description": "List all open buffers in Neovim",
@@ -952,6 +990,28 @@ pub async fn run_mcp_loop(app_name: &str, app_version: &str) {
                                 send_error(id, -32602, "Missing 'line' or 'message'").await;
                             }
                         }
+                        "nvim_set_extmark" => {
+                            if let (Some(line), Some(text)) = (
+                                args.get("line").and_then(serde_json::Value::as_i64),
+                                args.get("text").and_then(|v| v.as_str()),
+                            ) {
+                                let hl_group = args
+                                    .get("highlight_group")
+                                    .and_then(|v| v.as_str())
+                                    .unwrap_or("Comment");
+                                match set_nvim_extmark(line, text, hl_group).await {
+                                    Ok(()) => {
+                                        send_text_result!(
+                                            id.clone(),
+                                            format!("Successfully set extmark on line {}", line)
+                                        );
+                                    }
+                                    Err(e) => send_error(id, -32603, &e).await,
+                                }
+                            } else {
+                                send_error(id, -32602, "Missing 'line' or 'text'").await;
+                            }
+                        }
                         "nvim_list_buffers" => {
                             let lua_code = r"
                                 local bufs = vim.api.nvim_list_bufs()