docs(terminal-rendering): document plugin_install, commands_changed, compact_boundary rows

The renderer dispatch section was missing entries for three system
subtypes that now have specific renderers (added in the preceding
commits on this branch). Update the row taxonomy table and the
numbered dispatch description to cover all four system-subtype
handlers (thinking_tokens was already documented; 1b–1e are new).

docs/terminal-rendering.md

@@ -67,6 +67,10 @@ parent's negative pull.
 | `.tool-result-block.error` `<details>` | `Nl · headline` | red, body is text | long error `tool_result` (`is_error: true`); wrapper stripped | stream-json |
 | `.tool-use` | `⌁  task <id> started · <desc> [type]` | cyan | claude Task-tool subagent start (dead path — `Task` omitted from agent allow-list) | `renderTaskEvent` |
 | `.turn-end-ok` / `.turn-end-fail` / `.tool-result` | `⌁  task <id> ✓/✗/◌ <status> · <desc> · → <output_file>` | green / red / muted | claude Task-tool result (dead path for agents) | `renderTaskEvent` |
+| `.note` | `· ⚙ plugin install · loading…` or `✓ done` | muted | `system/plugin_install` (`status` = started/completed) | stream-json |
+| `.note` | `· ⚙ commands changed · N available` (expandable list of `/name` entries) | muted | `system/commands_changed` (slash-command set updated, usually post-plugin_install) | stream-json |
+| `.note` | `· ⚙ compact · <trigger> · <pre>→<post> tokens · <dur>` | muted | `system/compact_boundary` (compaction completed; metadata includes pre/post token counts, duration, trigger) | stream-json |
+| `.note` | `· ⚙ <subtype>` | muted | other `system` subtypes (context_window_exceeded, etc.) | stream-json catch-all |
 | `.note` | `· <text>` | muted | harness chatter | `LiveEvent::Note` |
 | `.note.stderr` | `! stderr: <line>` | amber/orange | stderr lines off claude | `LiveEvent::Note` (`text` starts `stderr:`) |
 | `.note.op` | `· operator: <text>` | mauve italic | operator-initiated notes (/cancel, /compact, /model, new-session) | `LiveEvent::Note` (`text` starts `operator:`) |
@@ -95,6 +99,19 @@ suffix, so the terminal degrades cleanly against older event shapes.
    while it's still the last one rendered (`nextElementSibling ==
    null`); any other event after it makes the next tick start a
    fresh row. Avoids a note-per-tick scrollback flood.
+1b. `system/plugin_install` → muted note `⚙ plugin install · loading…`
+   (on `started`) or `⚙ plugin install · ✓ done` (on `completed`).
+   Emitted in pairs: started fires before the plugin loads, completed
+   fires when it's ready. The `uuid` links the pair.
+1c. `system/commands_changed` → collapsible `.note` details row showing
+   the new slash-command count (`⚙ commands changed · N available`).
+   Expanding reveals each `/name` and its aliases. Fires after
+   `plugin_install` when a plugin registers new commands.
+1d. `system/compact_boundary` → muted note showing compaction summary:
+   `⚙ compact · <trigger> · <pre>→<post> tokens · <dur>`. Fields are
+   guarded individually — a missing field is silently omitted. Trigger
+   is `"manual"` (operator `/compact`) or `"auto"`.
+1e. Other `system/` subtypes → muted note `⚙ <subtype>`.
 2. `subtype == "task_started" | "task_notification"` →
    `renderTaskEvent` (subagent activity gets the `⌁` glyph).
 3. `type == "assistant"` → walk `message.content[]`:

frontend/packages/agent/src/app.js

@@ -1809,6 +1809,55 @@ 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;
+        }
+        // compact_boundary: claude completed a compaction pass. The metadata
+        // carries pre/post token counts, duration, and the trigger (manual vs
+        // auto). Show a single summary line so the operator can gauge how much
+        // context was shed.
+        if (v.subtype === 'compact_boundary') {
+          const m = v.compact_metadata || {};
+          const parts = ['⚙ compact'];
+          if (m.trigger) parts.push(m.trigger);
+          if (m.pre_tokens != null && m.post_tokens != null) {
+            const fmtTok = (n) => n >= 1_000_000 ? (n / 1_000_000).toFixed(1) + 'M'
+                                : n >= 1_000      ? Math.round(n / 1000) + 'k'
+                                :                   String(n);
+            parts.push(fmtTok(m.pre_tokens) + '→' + fmtTok(m.post_tokens) + ' tokens');
+          }
+          if (m.duration_ms != null) {
+            const ms = m.duration_ms;
+            parts.push(ms < 1000 ? ms + 'ms' : (ms / 1000).toFixed(1) + 's');
+          }
+          api.row('note', parts.join(' · '));
+          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.