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.

src/snapshots/rdbms_playground__ui__tests__populated_with_table_dark.snap

@@ -1,10 +1,10 @@
 ---
 source: src/ui.rs
-assertion_line: 1771
+assertion_line: 1841
 expression: snapshot
 ---
 ╭ Tables ──────────────────╮╭ Output ──────────────────────────────────────────╮
-│Customers                 ││[system] [ok] create table Customers              │
+│Customers                 ││[simple] create table Customers ✓                 │
 │Orders                    ││[system]   Customers                              │
 │                          ││[system]     id serial [PK]                       │
 │                          ││[system]     Name text                            │