M4b task 15: fix task 14's dropped refusal reason, as a listed edit

Task 15's session found the defect in serve/approvals.rs, a file it did not list, and stopped.
The comment in task 14 used one name for two values; task 14's tests could not reach it. Lesson T30.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M4b/README.md

@@ -65,6 +65,20 @@ signature (tip T29).
 
 At the end of task 15: about 845 tests (768 before task 01).
 
+## Changes during the run
+
+- 2026-09-24, task 15: tasks 01 to 14 committed, each on its first session. Task 15's session made
+  every change the task asks for, and one given test still failed: a refusal's reason was missing
+  from the post. It traced the cause to task 14's `ended` in `serve/approvals.rs`, a file task 15
+  did not list, and stopped instead of editing it, which was right. The fault was the plan's: in
+  `ended`, the refusal's text and a `DenyReason` were both called `reason`, and the comment said
+  `Refused -> Ended::Refused { by, reason }` without saying which; and task 14's own tests could
+  not send a refusal, so nothing there could catch it. Task 14 took the `DenyReason` reading, then
+  silenced the unused-variable warning on the other with `reason: _` (and said so in its log).
+  Task 15 now lists `serve/approvals.rs` with the exact fix. Checked in a scratch copy: the saved
+  attempt (`.state/runs/M4b/15-first-attempt*`) plus that fix passes every suite three times and
+  the gate, 845 tests. Resume from task 15.
+
 ## Running it
 
 ```sh