docs/plans/CHECKLIST.md: the checks before a plan is handed over; T41 to T43

One list for the design model to work through per task, gathering the
task-writing tips that apply before hand-over (T18, T19, T24 to T29,
T32 to T35, T39, T40) and three new ones from the combined Ornith
guidance: walk each given test by hand against the task's rules (T41),
grep earlier given files for anything the task changes (T42), and run
the gate's own checks on the given files (T43).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/implementer-lessons.md

@@ -78,6 +78,9 @@ How it is used:
 | T38 | A tool's answer that the model relays must say the outcome in plain words, including what did not happen ("not added to memory.md: …"). A label in parentheses reads as success. | M5a device check: the model twice told the owner "Added to memory.md" for a write that became an untrusted note. After task 22 it reported it correctly. |
 | T39 | Name the source directory of copied files in every task's Files section. | M5a task 06 logged a deviation about a files directory the task never named. |
 | T40 | Give the gate's shell timeout in the task (600000 ms). On a machine that also serves the model, the gate takes longer than the implementer's default of 2 minutes, and a timed-out gate looks like a failure. | M5a tasks 18 and 19 each ran the gate twice on titan; tasks 21 and 22, which gave the timeout, ran it once. |
+| T41 | Before hand-over, walk each given test by hand against the task's own rules. A replay of the reference proves the tests can pass; it cannot show that a test contradicts the task, because the reference was written to pass the test. | Not seen here yet; from the combined Ornith guidance (`~/projects/ornith/GUIDANCE.md` §5): crossbar's given tests pinned event positions and a tie-break no implementation following the task could meet. Listed in `docs/plans/CHECKLIST.md`. |
+| T42 | Before hand-over, grep every earlier given file and fixture for anything the task changes (strings, sentences, JSON shapes, counts). | T19 is one case of it (M3a `strict.rs`); crossbar had five such breaks (Ornith guidance §5). M5a task 22 handled it by keeping `edit.rs-task06` and `serve.rs-task10`. |
+| T43 | Run the gate's own checks on the given files themselves (`cargo fmt --check`, the line limit, clippy on test targets) before hand-over. A given file that fails the gate stalls the run, and the implementer may not edit it. | M4a task 16: a given test copied into the plan before `cargo fmt` came back reformatted (T29); crossbar stalled on given files that failed its gate (Ornith guidance §5). |
 
 ## What worked and should be kept
 

docs/plans/CHECKLIST.md

@@ -0,0 +1,62 @@
+# Before a plan is handed over
+
+The design model works through this list for every task before a plan goes to the implementer.
+Each item exists because of a recorded failure; the tip numbers point to
+`docs/implementer-lessons.md`, and "Ornith guidance" to the combined assessment of three projects
+(`~/projects/ornith/GUIDANCE.md`, section 5), whose crossbar findings are not in this repository's
+log.
+
+## The reference
+
+- [ ] The reference is replayed through `make gate` at each task's end state, on the toolchain of
+      the host that will run the plan (T34), in a scratch worktree that builds into its own
+      `target/` (T35). If a worktree ever shared the main `target/`, run `cargo clean` in the main
+      tree before the run.
+- [ ] The reference is broken on purpose one line at a time, and every break that can change
+      behaviour is caught by the given tests (T28). Record the count in the plan's README.
+- [ ] Each task file is read against the reference for anything the reference has that the task
+      does not ask for (T26).
+- [ ] The reference is moved off this machine (a git bundle on straylight) and its local branch
+      and worktree deleted before the run (T18).
+
+## The given files
+
+- [ ] Each given test fails against the tree before the task for exactly the reasons the task
+      names, and the task lists those tests by name.
+- [ ] **Walk each given test by hand against the task's own rules.** A replay cannot catch a test
+      that no implementation following the task could pass (Ornith guidance: crossbar's pinned
+      event positions and tie-breaks).
+- [ ] **Grep every earlier given file and fixture** for anything this task changes: strings,
+      JSON shapes, sentences, counts. A later task that changes a shared file gives the earlier
+      task its own copy (`name-taskNN`, T19; Ornith guidance: five breaks in crossbar).
+- [ ] **Run the gate's own checks on the given files**: `cargo fmt --check`, the 500-line limit,
+      clippy on test targets. A given file that fails the gate stalls the run (Ornith guidance).
+- [ ] No test depends on timing: every wait has a deadline on observable state (T27), no test
+      asserts the order of work done on separate threads (T32), and nothing sleeps instead of
+      waiting. Stress any test that ever flaked under load.
+- [ ] No file the implementer reads holds the model's chat-template tags whole (T33): grep the
+      plan, the given files and the skeletons for them.
+
+## The task text
+
+- [ ] Every quoted "before" block is found exactly once in the tree at that task's start, and every
+      "after" block is in the reference.
+- [ ] New code is a compiling skeleton; no `todo!()` has a comment longer than about six lines
+      (T25); every comment is read beside its signature (T29); conversions and accessors are
+      named (`u16::try_from`, `.get(i)`).
+- [ ] A call table lists every API the task uses, with signatures (T25).
+- [ ] Environment facts the implementer would otherwise go looking for are stated (OS limits,
+      library quirks, where a file lives). For a client of another program, its contract is
+      pasted in: a saved response or a schema fragment per call (Ornith guidance).
+- [ ] The files section names the source directory of every copied file (T39).
+- [ ] The steps give the gate's shell timeout, 600000 ms (T40), and `Cargo.lock` is in the
+      `git add` line (T24).
+- [ ] The commit subject is given exactly. The trailer comes from `tools/run-plan.sh`.
+- [ ] "Stop and report if" names the specific conditions: a quoted line not found, a different set
+      of failing tests, any other test failing, a comment that does not fit its signature.
+
+## The run
+
+- [ ] The plan README gives the command, with `OPENCODE_MODEL`, and the expected test count after
+      each task.
+- [ ] The working tree is clean, the reference is off the machine, and the OpenCode TUI is closed.