# M4b task 16: `brokerd`'s refusal without a runner says why

**Branch:** `m4b` (run `git switch m4b`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `brokerd: the refusal without a runner names the missing [runner] section`

## Goal

When `brokerd.toml` has no `[runner]` section, every allowed call is answered with the fixed
sentence `REFUSING`. It still says `the runner arrives in M3b`, which stopped being true when M3b
merged: the model, and the owner reading its answer, are told to wait for something that has
already happened. Found in the M4b device check (review of 2026-09-24, finding 2). The sentence
becomes:

```text
no tool runner is configured: brokerd.toml has no [runner] section
```

It is still a fixed sentence (it reaches the model), so nothing else changes.

## Files

- Copy: a **new version** of the given `crates/brokerd/tests/runner.rs` (one assertion: the new
  sentence)
- Modify: `crates/brokerd/src/runner.rs`, `docs/implementer-log.md`
- Do **not** change `crates/loopd/tests/broker_port.rs`: it has the old sentence as test data
  only, and `loopd` never checks it.

## The change, exactly

In `crates/brokerd/src/runner.rs`, the line

```rust
pub const REFUSING: &str = "the runner arrives in M3b";
```

becomes

```rust
pub const REFUSING: &str = "no tool runner is configured: brokerd.toml has no [runner] section";
```

The doc comment above it stays as it is.

## Steps

- [ ] **1. Copy.** `git switch m4b`, then
  `cp docs/plans/M4b/files/crates/brokerd/tests/runner.rs crates/brokerd/tests/`
- [ ] **2. See it fail.** `cargo test -p brokerd --test runner`. Expected: one test fails,
  `the_production_runtime_refuses_every_call`, on the sentence.
- [ ] **3. Make the change.** Run `cargo fmt --all`.
- [ ] **4. See it pass.** `cargo test -p brokerd --test runner`: every test passes. Then
  `cargo test -p brokerd`: every suite passes.
- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`, with the same number of
  tests as before this task (about 846).
- [ ] **6. Log and commit.** `git add crates/brokerd docs/implementer-log.md Cargo.lock && git commit`

## Done when

- `cargo test -p brokerd` passes; `make gate` prints `gate: ok`.

## Stop and report if

- Any other test fails because of the sentence. Do not change that test.