Decide where reference implementations are still written

From M3a: a full reference for the audit chain and the approval
handoff, the property test's oracle for policy, and compiling
skeletons for everything else. Each plan records what its checks
exposed, and that record decides what the next milestone gets.
Tip T17; the spec's handover section says which check each module has.

Also restores three / examples that an editing tool had decoded
to a plain slash.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/implementer-lessons.md

@@ -49,6 +49,7 @@ How it is used:
 | 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. |
+| T17 | Match the check to the risk. A full reference for intricate logic whose writing debugs the spec (state machines, concurrency); a naive oracle inside the test for decision logic; a compiling skeleton (`todo!()` bodies under the real signatures) for plumbing. Record what each check exposed, and let the record decide what the next milestone gets. | Across M1 to M2b the references caught no implementer defect. They caught task defects (T16) and missed what they shared with the tests (T5). Decision of 2026-09-18. |
 
 ## What worked and should be kept