Review M2b task 11: accept; M2b done

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/implementer-lessons.md

@@ -48,6 +48,7 @@ How it is used:
 | T13 | Set the sampling defaults on the server for coding agents (`temp`, `top-p`, `top-k`, `reasoning-budget`). OpenCode sends none, so the server's default temperature of 1.0 applies, and thinking has no cap. | Five Ornith turns ran 16k tokens of thinking to the output limit and produced nothing. |
 | T14 | When a rule has a "before the final frame" or "on every exit" shape, list the exits in the task, or say "including the early returns". The implementer follows the path the task walks through. | M2b finding 1: task 07 walked the success path and the turn's error; the open and create errors were early returns it did not mention. |
 | T15 | Running the whole plan through `tools/run-plan.sh` worked: ten tasks, one commit each, unattended, in about three hours. Keep the TUI closed while it runs; a second message into the driver's session starts a second agent on the same tree. | M2b run, 2026-09-18. |
+| T16 | When a task prescribes the fix, check that it compiles against the types as they are, in the reference tree, before handing it over. A fix that cannot be written as described pushes the implementer outside the listed paths. | M2b task 11: "`unwrap_or_else` with a fixed valid id" had no non-panicking form outside `proto`; the implementer added `Default` to `SessionId` and reported it. |
 
 ## What worked and should be kept
 
@@ -84,4 +85,5 @@ M1, and failed as a coordinator. First-gate pass rate for the milestone: 8 of 13
 M2b was Ornith alone, driven by the shell script: ten tasks, ten commits, no intervention, first
 gate 4 of 10, and four low findings in review, two of which are the "everywhere" pattern again in
 a new shape (paths instead of types). Its code reads well and follows the numbered steps in the
-tasks closely; where the task did not walk a path, the rule was not applied there.
+tasks closely; where the task did not walk a path, the rule was not applied there. Task 11 fixed all four in one run and reported its one
+deviation, which the task itself had caused.