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>

AGENTS.md

@@ -9,7 +9,9 @@ Unix sockets. You are implementing it one task at a time.
    nothing else. Do not start the next task.
 2. Do the steps in order. Where a step shows a command and its expected output, run it and compare.
 3. Do not read `docs/design.md` or other plans unless the task tells you to. The task file quotes
-   what you need.
+   what you need. Do not read `docs/implementer-log.md` until the log-and-commit step, and then
+   only the first 12 lines (the title through the header row). The rest of that file will fill
+   the session.
 4. If something in the task is impossible, contradictory, or fails twice in the same way, **stop**.
    Do not improvise, do not change a test, do not weaken a check. Add your row to
    `docs/implementer-log.md` with status `stopped`, say what you tried and what happened, commit
@@ -97,7 +99,7 @@ These come from defects found in review. The evidence is in `docs/implementer-le
   anchor the edit on another line.
 - Write your log row from `git diff` after staging, naming only what the diff shows.
 - Add the log row immediately under the table header (newest first). Do not insert it by date
-  among older rows.
+  among older rows. Read only the first 12 lines of `docs/implementer-log.md` to do that.
 - After `gate: ok`, do not run `cargo test --workspace` only to count tests. If HEAD already has
   this task's commit and a `done` row, stop.
 
@@ -131,10 +133,11 @@ Useful while working: `cargo test -p <crate> --test <file>` runs one test file,
 
 ## The implementer log
 
-Before you commit, add one row immediately under the header of the table in
-`docs/implementer-log.md` (newest first) and include the file in the commit. Do not insert the
-row by date among older rows. Be honest: the log is how the owner judges the process, and a wrong
-row is worse than a bad one.
+Before you commit, read only the first 12 lines of `docs/implementer-log.md` (title through the
+header row). Insert one row immediately under the header (newest first) and include the file in
+the commit. Do not insert the row by date among older rows. Do not read the rest of the file: it
+is large enough to fill the session. Be honest: the log is how the owner judges the process, and
+a wrong row is worse than a bad one.
 
 | Column | What to write |
 |---|---|