feat: replace the [ok] summary line with a ✓/✗ echo marker

An audit of the command surface found the `[ok] <verb> <subject>`
summary line duplicated the echo line above it everywhere; its only
unique signal was success-vs-error. Retire it: a command's echo line
now resolves from `running: <input>` to `<input> ✓` / `<input> ✗`
on completion, and the symmetric `"<verb> <subject>" failed:` prefix
is dropped (only the reason remains). Content lines (row counts,
structure, plan tree, teaching echo) are unchanged.

Echo lines carry an EchoStatus; executed commands push Pending and
resolve the oldest-pending echo on their result event (FIFO worker —
correct under interleaving). Parse-time and pre-flight rejections are
not executed and keep their running: + caret rendering. App-command
[ok] lines (rebuild/export/replay) are payload-bearing and untouched.
ADR-0040.

tests/walking_skeleton.rs

@@ -306,9 +306,11 @@ fn create_table_flow_updates_tables_list_and_structure_view() {
         rendered.contains("Customers"),
         "items panel should list Customers:\n{rendered}"
     );
+    // ADR-0040: success is the ✓ marker on the command's echo line
+    // (the `[ok] create table Customers` summary line was retired).
     assert!(
-        rendered.contains("[ok] create table Customers"),
-        "output should confirm success:\n{rendered}"
+        rendered.contains("create table Customers with pk ✓"),
+        "the command echo should resolve to a success marker:\n{rendered}"
     );
     // The structure table renders one line per column; the
     // `id` row shows both the name and its `serial` type
@@ -397,7 +399,12 @@ fn drop_table_flow_clears_items_list() {
     assert!(app.current_table.is_none());
     let rendered = rendered_text(&mut app, &Theme::dark(), 80, 24);
     assert!(rendered.contains("(none yet)"));
-    assert!(rendered.contains("[ok] drop table Customers"));
+    // ADR-0040: `drop table` is content-less, so the echo's ✓ marker
+    // is the entire success signal (replacing `[ok] drop table …`).
+    assert!(
+        rendered.contains("drop table Customers ✓"),
+        "the drop echo should resolve to a success marker:\n{rendered}"
+    );
 }
 
 #[test]