docs: M5d-2: read only the implementer-log header

Task 06 opened the whole implementer log (~360 kB) after reading supporting
modules; OpenCode returned "Context size has been exceeded" and made no
commit. AGENTS.md and tasks 06-10 now say to read the first 12 lines only.

Co-Authored-By: Grok 4.6 <noreply@x.ai>

docs/implementer-lessons.md

@@ -101,6 +101,7 @@ How it is used:
 | T59 | After `gate: ok`, the task is done counting. Do not tell the implementer to run `cargo test --workspace` (or the gate) again only to compare a number. AGENTS.md already forbids a second gate "only to count"; say the same of the workspace test run. | M5c-2: all twelve sessions ran `cargo test --workspace` after a passing gate to rediscover the total, even with T50's wording in the task. |
 | T60 | The driver must not truncate a task's run log, and must not start a task whose commit and `done` row are already in HEAD. A power-loss resume continues the existing log file; it does not open it for write from offset zero, and it skips a task that is already committed. | M5c-2 task 05: `.state/runs/M5c2/05-memoryd-honcho-readers.log` is four `=== …` resume sections after HEAD was at task 12; the original transcript is gone. Commit `b370c60` was already complete. |
 | T61 | Name where the log row goes: immediately under the header, newest first. The implementer otherwise inserts by date among older rows. | M5d-1 task 06. |
+| T62 | The implementer log is hundreds of kilobytes. A task that needs a row must say to read only the first 12 lines (title through the header). Reading the whole file exceeds the session context and the driver reports "made no commit". | M5d-2 task 06: after reading the task and supporting modules it opened `docs/implementer-log.md` and OpenCode returned `Context size has been exceeded.`; HEAD did not move. |
 
 ## What worked and should be kept