Accept M1 follow-ups; add implementer lessons and promote four rules to AGENTS.md

Tasks 08 and 09 pass the checklist and the reviewer's probes, so M1 is
complete. docs/implementer-lessons.md turns the review findings into
tips for the implementer and for task writing, each tied to its
evidence, with a column to record whether the defect comes back.

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

AGENTS.md

@@ -39,6 +39,18 @@ Unix sockets. You are implementing it one task at a time.
 - Keep struct fields and enum variants in the order the task gives. The order is the wire format.
 - Comments say why, not what. Match the amount of commenting you see in the task's examples.
 
+## Lessons from earlier reviews
+
+These come from defects found in review. The evidence is in `docs/implementer-lessons.md`.
+
+- When a rule says "every" or "everywhere", finish by listing each place it could apply (every
+  struct and enum in the file, every script, every branch) and check them one by one. An example
+  in a task shows one place; the rule covers all of them.
+- A check must fail when it cannot do its job: missing input, unreadable file, a tool that errors.
+  Never throw errors away with `2>/dev/null`, `|| true` or an ignored `Result`.
+- Report every problem you find, not only the first.
+- Log every attempt, including one you abandon.
+
 ## The gate
 
 `make gate` must print `gate: ok` before a task is done. It runs offline: rustfmt, clippy with