feat(agent-term): show details for plugin_install and commands_changed

Currently both system subtypes fall through to the generic muted
note renderer (⚙ <subtype>), giving the operator no insight into
what is happening.

plugin_install:
  Render status explicitly — "loading…" on started, "✓ done" on
  completed — so a slow plugin boot is visible in the scrollback
  instead of two identical cryptic rows.

commands_changed:
  Render the count of available slash commands in the summary row
  and expand to the full list in a collapsible details block. The
  list is most useful right after a fresh plugin_install so the
  operator can see exactly which /commands are now on offer.

Both handlers sit immediately before the generic catch-all in the
system-subtype dispatch in renderStream, preserving the existing
fall-through for other subtypes (context_window_exceeded etc.).

Closes #2183.

frontend/packages/agent/src/app.js

@@ -1809,6 +1809,34 @@ window.marked = marked;
           }
           return;
         }
+        // plugin_install: claude is loading/finishing a plugin (MCP server or
+        // slash-command provider). Show the status so the operator knows when
+        // a fresh session is loading its toolset.
+        if (v.subtype === 'plugin_install') {
+          const status = v.status === 'completed' ? '✓ done'
+                       : v.status === 'started'   ? 'loading…'
+                       :                            (v.status || '?');
+          api.row('note', '⚙ plugin install · ' + status);
+          return;
+        }
+        // commands_changed: the set of available slash commands changed (usually
+        // right after plugin_install). Show the count in the summary; expand to
+        // see the full list.
+        if (v.subtype === 'commands_changed') {
+          const cmds = Array.isArray(v.commands) ? v.commands : [];
+          if (!cmds.length) {
+            api.row('note', '⚙ commands changed · (empty)');
+            return;
+          }
+          const summary = '⚙ commands changed · ' + cmds.length + ' available';
+          const body = cmds.map((c) => {
+            const aliases = c.aliases && c.aliases.length
+              ? '  [/' + c.aliases.join(', /') + ']' : '';
+            return '/' + c.name + aliases;
+          }).join('\n');
+          api.details('note', summary, body);
+          return;
+        }
         // Other system subtypes (context_window_exceeded, etc.) — render a
         // muted note with the subtype label; reserve the loud orange `sys`
         // catch-all for truly unrecognised top-level types.