subagent headers: leave out a model or effort a header cannot hold

The values come from the agent's start request. One with a line break made
async-nats panic inside the publish task, which would have stopped every
subagent's rows. They are now trimmed, and skipped when empty, over 64
characters, or not a valid header value.

hive-subagent-mcp/src/swarm_term.rs

@@ -84,19 +84,36 @@ pub struct RunInfo {
 }
 
 impl RunInfo {
-    /// The headers for one row: only the values there are.
+    /// The headers for one row: only the values there are, and only ones a
+    /// header can hold. The values come from the agent's start request, so one
+    /// with a line break (which `async_nats` panics on) or an absurd length is
+    /// left out rather than published.
     fn headers(&self) -> async_nats::HeaderMap {
         let mut headers = async_nats::HeaderMap::new();
-        if let Some(model) = self.model.as_deref().filter(|v| !v.is_empty()) {
+        if let Some(model) = header_value(self.model.as_deref()) {
             headers.insert(subagent_term::HEADER_MODEL, model);
         }
-        if let Some(effort) = self.effort.as_deref().filter(|v| !v.is_empty()) {
+        if let Some(effort) = header_value(self.effort.as_deref()) {
             headers.insert(subagent_term::HEADER_EFFORT, effort);
         }
         headers
     }
 }
 
+/// The longest model or effort name published. Real ones are a few
+/// characters; the value rides on every row and is shown in the UI.
+const MAX_VALUE_LEN: usize = 64;
+
+/// `value` trimmed, as a header value, or `None` when it is empty, too long, or
+/// not one a header can carry.
+fn header_value(value: Option<&str>) -> Option<async_nats::HeaderValue> {
+    let value = value?.trim();
+    if value.is_empty() || value.len() > MAX_VALUE_LEN {
+        return None;
+    }
+    value.parse().ok()
+}
+
 struct Line {
     subagent: String,
     info: Arc<RunInfo>,
@@ -486,6 +503,30 @@ mod tests {
         assert!(info.headers().is_empty());
     }
 
+    /// The values are the caller's. A line break would panic `async_nats`, in
+    /// the task that publishes every subagent's rows.
+    #[test]
+    fn a_value_a_header_cannot_hold_is_left_out_and_never_panics() {
+        for bad in ["a\nb", "high\r\nX: y", &"x".repeat(MAX_VALUE_LEN + 1)] {
+            let info = RunInfo {
+                model: Some(bad.to_owned()),
+                effort: Some(bad.to_owned()),
+            };
+            assert!(info.headers().is_empty(), "{bad:?}");
+        }
+        let padded = RunInfo {
+            model: Some(" opus\n".to_owned()),
+            effort: None,
+        };
+        assert_eq!(
+            padded
+                .headers()
+                .get(subagent_term::HEADER_MODEL)
+                .map(ToString::to_string),
+            Some("opus".to_owned())
+        );
+    }
+
     #[test]
     fn every_kind_of_line_renders_as_stamped_rows() {
         let mut ctx = ClassifyCtx::default();