M4b review, and follow-up tasks 16 to 18

The review of tasks 01 to 15 goes in the implementer log, with the
device check against the owner's Mattermost. Three follow-ups:

- 16: the refusal without a runner says why.
- 17: the texts that say who asked.
- 18: approvals from non-approvers in direct or group messages go to
  an approver.

Task 14's given serve_approvals.rs becomes serve_approvals.rs-task14,
since task 18 hands over a new version. Tip T31 is added.

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

docs/plans/M4b/16-brokerd-refusing-message.md

@@ -0,0 +1,63 @@
+# 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.