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.

docs/requirements.md

@@ -135,8 +135,10 @@ handoff-14 cleanup; 449 after B2/C2.)
   belongs, not in persistent state). The runtime gates the
   ADR-0030 §10 teaching echo on it: a DSL-form command run in
   advanced/one-shot mode renders the equivalent advanced-mode SQL
-  beneath `[ok]`; simple-mode and SQL-entered submissions stay
-  silent. Echo coverage: **ADR-0038 is feature-complete** — every
+  beneath the command's echo line (the `[ok]` summary it once sat
+  under was retired by ADR-0040, issue #9 — the echo line now carries
+  a ✓/✗ completion marker instead); simple-mode and SQL-entered
+  submissions stay silent. Echo coverage: **ADR-0038 is feature-complete** — every
   catalogue row in §7 round-trips per line through the advanced
   walker (the §1 copy-paste contract; §6 category 2 holds it per
   line), every §6 category-3 line surfaces, and the §4