host.sock: give HostResponse a wire-schema version, warn on mismatch

hive-host-sock/src/lib.rs

@@ -517,8 +517,27 @@ pub struct HiveUrls {
     pub matrix: Option<String>,
 }
 
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+/// Wire-schema version for [`HostResponse`]. Bump only for a breaking
+/// change — removing, renaming, or retyping an existing field; adding a
+/// new `Option`/`skip_serializing_if` field (the pattern every field on
+/// this struct already follows) is not breaking and needs no bump. A
+/// client compares this against its own compiled-in copy of the constant
+/// and warns (does not fail the request) on mismatch — see
+/// `hivectl::client::version_mismatch_hint`, the one place that reads it.
+pub const HOST_SOCK_VERSION: u32 = 1;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct HostResponse {
+    /// Wire-schema version this response was built against, see
+    /// [`HOST_SOCK_VERSION`]. `#[serde(default)]` so a pre-version daemon
+    /// (or a hand-built test fixture) deserializes as `0` rather than
+    /// failing to parse — `0` reads as "older than any real version" to a
+    /// mismatch check, which is the correct answer for wire predating
+    /// this field. Always populated on the write side: every constructor
+    /// below routes through `Self::default()`, whose `Default` impl (not
+    /// derived — see below) sets this to the current version.
+    #[serde(default)]
+    pub version: u32,
     pub ok: bool,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub error: Option<String>,
@@ -568,6 +587,31 @@ pub struct HostResponse {
     pub quota: Option<Vec<QuotaRow>>,
 }
 
+/// Hand-written rather than `#[derive(Default)]` for one reason:
+/// `version` must default to [`HOST_SOCK_VERSION`], not `u32::default()`
+/// (`0`). Every field below it is the same all-`None`/`false`/empty shape
+/// `derive(Default)` would have produced — only `version` differs — and
+/// every constructor in `impl HostResponse` builds through `..Self::default()`,
+/// so this is the one place that has to get it right.
+impl Default for HostResponse {
+    fn default() -> Self {
+        Self {
+            version: HOST_SOCK_VERSION,
+            ok: false,
+            error: None,
+            agents: None,
+            approvals: None,
+            urls: None,
+            agent_statuses: None,
+            agent_exists: None,
+            queued_dags: None,
+            nodes: None,
+            messages: Vec::new(),
+            quota: None,
+        }
+    }
+}
+
 impl HostResponse {
     #[must_use]
     pub fn success() -> Self {