feat(#2196): stamp _body on mcp__bash__run, drop tool-specific JS branch

Backend now stamps `_body` (full `$ <cmd>`) alongside `_summary` (first
line) and `_category: "rich"` for mcp__bash__run tool_use entries. The
frontend drops the mcp__bash__run-specific branch in renderRichToolUse
and uses a generic `if (c._body)` path instead — api.details() with the
backend-computed summary and body, no JS knowledge of the tool name.

frontend/packages/agent/src/app.js

@@ -1559,14 +1559,11 @@ window.marked = marked;
           'answer #' + id + (lines > 1 ? ` · ${lines}L` : ''),
           a, icon);
       }
-      // Bash task runner — show full command in an expandable pre block so
-      // multi-line scripts are readable. Summary uses the first line so the
-      // row is identifiable without expanding.
-      if (name === 'mcp__bash__run') {
-        const cmd = String(input.cmd || '');
-        const firstLine = cmd.split('\n')[0];
-        const summary = 'run* $ ' + trim(firstLine.trim(), 72);
-        return api.details('tool-use', summary, '$ ' + cmd, icon);
+      // Generic plain-text body: the backend stamps `_body` for tools whose
+      // expandable content is plain text (e.g. mcp__bash__run full command).
+      // Render as a collapsible details row without any tool-specific logic.
+      if (c._body) {
+        return api.details('tool-use', c._summary || name || '?', c._body, icon);
       }
       return null;
     }

hive-agent/src/stream_enrich.rs

@@ -230,6 +230,7 @@ fn enrich_tool_use_entry(entry: &mut Value) {
     let icon = tool_icon(&name);
     let summary = fmt_tool_use(&name, &input);
     let rich = is_rich_tool(&name);
+    let body = rich_tool_body(&name, &input);
     let Some(obj) = entry.as_object_mut() else {
         return;
     };
@@ -238,6 +239,9 @@ fn enrich_tool_use_entry(entry: &mut Value) {
     if rich {
         obj.insert("_category".to_owned(), json!("rich"));
     }
+    if let Some(b) = body {
+        obj.insert("_body".to_owned(), json!(b));
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -260,6 +264,29 @@ fn is_rich_tool(name: &str) -> bool {
     )
 }
 
+/// Pre-compute the expandable body text for tools that have one.
+///
+/// Returns `Some(body)` when the tool has a meaningful multi-line body that
+/// the frontend can display in a `<details>` block without tool-specific JS.
+/// `None` for tools whose body is computed client-side (diffs, markdown) or
+/// that have no body at all.
+fn rich_tool_body(name: &str, input: &Value) -> Option<String> {
+    match name {
+        // Full bash command — first line is already in `_summary`; the full
+        // `cmd` (prefixed with `$ `) is the body so multi-line scripts are
+        // readable on expand.
+        "mcp__bash__run" => {
+            let cmd = input.get("cmd").and_then(Value::as_str).unwrap_or("");
+            if cmd.is_empty() {
+                None
+            } else {
+                Some(format!("$ {cmd}"))
+            }
+        }
+        _ => None,
+    }
+}
+
 fn tool_icon(name: &str) -> &'static str {
     // Exact-match table first, then prefix/contains fallbacks.
     match name {