Plan M4b: approvals over Mattermost in 15 tasks

Skeletons for new files only; existing files change by exact edits, so no task overwrites an
earlier one's work. Each task's tests were run against a reference built in order from master,
with the gate at each step and mutations checked; each skeleton compiles and fails its tests, and
every todo!() comment was read beside its signature. The reference is kept off this machine.

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

docs/plans/M4b/files/crates/brokerd/tests/config_gateway.rs

@@ -0,0 +1,43 @@
+//! `[gateway]` in `brokerd.toml`: absent means Mattermost approvals are off; an empty socket is
+//! the default under `home` (M4b spec, section 3). Do not edit.
+
+use std::path::PathBuf;
+
+use brokerd::config::Config;
+
+fn parse(text: &str) -> Config {
+    Config::parse(&format!("[paths]\nhome = \"/srv/bx\"\n{text}")).unwrap()
+}
+
+#[test]
+fn without_the_section_approvals_stay_in_bxctl() {
+    let config = parse("");
+    assert_eq!(config.gateway, None);
+    assert_eq!(config.gateway_socket(), None);
+}
+
+#[test]
+fn an_empty_section_or_socket_is_the_default() {
+    let default = Some(PathBuf::from("/srv/bx/run/gateway/gateway.sock"));
+    assert_eq!(parse("[gateway]\n").gateway_socket(), default);
+    assert_eq!(
+        parse("[gateway]\nsocket = \"\"\n").gateway_socket(),
+        default
+    );
+}
+
+#[test]
+fn a_socket_path_is_used_as_given() {
+    let config = parse("[gateway]\nsocket = \"/run/bx/gw.sock\"\n");
+    assert_eq!(
+        config.gateway_socket(),
+        Some(PathBuf::from("/run/bx/gw.sock"))
+    );
+}
+
+#[test]
+fn unknown_keys_are_errors() {
+    assert!(Config::parse("[gateway]\nsockets = \"/x\"\n").is_err());
+    assert!(Config::parse("[gateway]\nsocket = \"/x\"\nenabled = true\n").is_err());
+    assert!(Config::parse("[gatewy]\nsocket = \"/x\"\n").is_err());
+}