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/plans/M5d2/06-loopd-scheduler-heartbeat.md

@@ -38,6 +38,12 @@ Tests call `tick(ctx, now)` with injected time. No sleeps (T27).
   `crates/loopd/src/main.rs`, `docs/implementer-log.md`.
 - Do not touch: any other file.
 
+**Read only what the step names.** Step 1 is the `cp`. Do not open `docs/implementer-log.md`,
+`docs/runbook.md`, or `crates/loopd/src/{turn,session,compact,baseline,config}.rs` to start.
+The facts table is complete. At step 7, read only the first 12 lines of
+`docs/implementer-log.md` (title through the header row) and insert the new row immediately
+under the header. The rest of that file fills the session (~360 kB).
+
 ## Facts you need
 
 - **`Context::hold`.** Same insert as `handle`: if the id is already in `busy`, `Err(Busy)`;
@@ -199,7 +205,8 @@ pass so it runs once.
 - [ ] **6. Run the gate.** `cargo fmt --all`, then `make gate`, timeout 600000 ms. Last line
   `gate: ok`. Test count **1551** (1545 before, plus 6). After `gate: ok`, do not add up
   `test result:` lines.
-- [ ] **7. Log and commit.** Task cell `M5d2/06-loopd-scheduler-heartbeat`. Stage with
+- [ ] **7. Log and commit.** Task cell `M5d2/06-loopd-scheduler-heartbeat`. Read only the first
+  12 lines of `docs/implementer-log.md`; insert the row under the header. Stage with
   `git add crates/loopd docs/runbook.md docs/implementer-log.md Cargo.lock`, then commit as
   `AGENTS.md` says.