M8: MCP and control calls name the document they act on (#998) (#1533)

Tools that act on a document, and the control methods engine.execute,
document.inspect, node.get and ui.render, take an optional `document`
(an open file's id from doc.list / document.inspect, or its path). The
call acts on that file and the tab in front stays in front, so an
attached agent keeps editing its own file when the user switches tabs.

crates/engine/src/cmd/file.rs

@@ -318,15 +318,21 @@ pub fn specs() -> Vec<CommandSpec> {
         }),
         spec!("doc.nextTab", "Next tab", "{}", false, |s, _| cycle_doc(s, 1)),
         spec!("doc.previousTab", "Previous tab", "{}", false, |s, _| cycle_doc(s, -1)),
-        spec!("doc.list", "List documents", "{}", false, |s, _| {
-            Ok(Value::Array(
+        spec!(
+            "doc.list",
+            "List documents",
+            "{}  → [{index, id, name, path, dirty, active}]: `id` names the file in a control or MCP call's `document`",
+            false,
+            |s, _| {
+                Ok(Value::Array(
                 s.docs
                     .iter()
                     .enumerate()
-                    .map(|(i, d)| json!({"index": i, "name": d.doc.name, "path": d.path, "dirty": d.is_dirty(), "active": i == s.active}))
+                    .map(|(i, d)| json!({"index": i, "id": d.uid, "name": d.doc.name, "path": d.path, "dirty": d.is_dirty(), "active": i == s.active}))
                     .collect(),
             ))
-        }),
+            }
+        ),
         spec!("doc.home", "Home", "{}", false, |s, _| {
             s.request_ui("home", json!({}));
             ok()