stream_enrich: document Write absence; skip -0 count in Edit summary

hive-agent/src/stream_enrich.rs

@@ -273,6 +273,12 @@ fn is_rich_tool(name: &str) -> bool {
 ///
 /// Returns `None` for tools whose body is inherently client-side (markdown
 /// rendering, DOM forms) or that have no body at all.
+///
+/// **`Write` is intentionally absent**: its `content` field can be megabytes
+/// and is one-sided (no `old_string` to diff against), making it too large to
+/// inline in every SSE event. The flat `"Write /path"` summary row is
+/// sufficient — the operator can open the file directly if they need to inspect
+/// the written content.
 fn rich_tool_body(name: &str, input: &Value) -> Option<(String, &'static str)> {
     match name {
         // Edit diff: old lines prefixed `- `, new lines prefixed `+ `.
@@ -396,19 +402,22 @@ fn fmt_builtin_tool(name: &str, short: &str, input: &Value) -> String {
         "Read" | "Write" => format!("{short} {}", sv(input, "file_path")),
         "Edit" => {
             let path = sv(input, "file_path");
-            let old_n = input
+            let old = input
                 .get("old_string")
                 .and_then(Value::as_str)
-                .unwrap_or("")
-                .lines()
-                .count();
+                .unwrap_or("");
             let new_n = input
                 .get("new_string")
                 .and_then(Value::as_str)
                 .unwrap_or("")
                 .lines()
                 .count();
-            format!("{short} {path} · -{old_n} +{new_n}")
+            if old.is_empty() {
+                format!("{short} {path} · +{new_n}")
+            } else {
+                let old_n = old.lines().count();
+                format!("{short} {path} · -{old_n} +{new_n}")
+            }
         }
         "Glob" | "Grep" => format!("{short} {}", sv(input, "pattern")),
         "Bash" => {