M4a task 16: read a secret file from the file that was checked (review finding 2)

The test swaps the file between the check and the open through a `between` hook. A reference fix
passed it and the gate (769 tests) in the working tree, caught two mutations, and was removed.

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

docs/plans/M4a/README.md

@@ -62,8 +62,9 @@ checked to compile against its task's tests and fail them.
 | 13 | `13-gatewayd-deliver.md` | turns on `loop.sock`, answers in the thread | `deliver.rs`, `support/fake_loop.rs` | reference; 11 mutations, 10 caught; the one left accepts an event frame with another id, which `loopd` never sends |
 | 14 | `14-gatewayd-serve.md` | the serve loop: routing, typing, catch-up, reconnecting | `serve.rs`, `serve_restart.rs`, `support/fake_mm.rs`, `support/gateway.rs` | reference; 14 mutations, all caught once four tests were added; 8 runs clean |
 | 15 | `15-gatewayd-main.md` | `gatewayd serve --config <path>` | `main.rs` | reference; run against the owner's server, token never printed |
+| 16 | `16-gatewayd-secret-file-race.md` | review finding 2: a secret file is read from the file that was checked | `secrets_race.rs` | reference in the working tree, then removed; 2 mutations, both caught; gate at the end state |
 
-At the end of task 15: about 762 tests (650 before task 01).
+At the end of task 15: about 762 tests (650 before task 01). The design model's review fixes add 2; at the end of task 16: about 769.
 
 ## Changes during the run
 
@@ -100,6 +101,9 @@ At the end of task 15: about 762 tests (650 before task 01).
   signature. Tasks 14 and 15 now say to stop and quote a comment that does not fit. The attempt is
   saved in `.state/runs/M4a/14-second-attempt*`. Resume from task 14.
 
+- 2026-09-24, after the review: the design model fixed findings 1 and 3 (commit `0081f24`), and
+  wrote task 16 for finding 2. Run it with `tools/run-plan.sh docs/plans/M4a 16`.
+
 ## Running it
 
 ```sh