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/14-gatewayd-serve-approvals.md

@@ -75,7 +75,7 @@ both first). To fill, each with its steps in its comment: `place`, `ended`, `mar
 ## Steps
 
 - [ ] **1. Copy.** `git switch m4b`, then
-  `cp docs/plans/M4b/files/crates/gatewayd/src/serve/approvals.rs crates/gatewayd/src/serve/ && cp docs/plans/M4b/files/crates/gatewayd/tests/serve_approvals.rs crates/gatewayd/tests/ && cp docs/plans/M4b/files/crates/gatewayd/tests/support/brokerd_client.rs docs/plans/M4b/files/crates/gatewayd/tests/support/fake_mm.rs crates/gatewayd/tests/support/`
+  `cp docs/plans/M4b/files/crates/gatewayd/src/serve/approvals.rs crates/gatewayd/src/serve/ && cp docs/plans/M4b/files/crates/gatewayd/tests/serve_approvals.rs-task14 crates/gatewayd/tests/serve_approvals.rs && cp docs/plans/M4b/files/crates/gatewayd/tests/support/brokerd_client.rs docs/plans/M4b/files/crates/gatewayd/tests/support/fake_mm.rs crates/gatewayd/tests/support/`
 - [ ] **2. Make the six `serve/mod.rs` changes.** `cargo build -p gatewayd` must succeed.
 - [ ] **3. See it fail.** `cargo test -p gatewayd --test serve_approvals`. Expected: 4 fail, 1
   passes (the socket is there while `gatewayd` runs).