feat: collapse tool calls into "See reasoning" disclosure on completion

While the agent is working, tool calls stream live so you can watch
progress. When the agent finishes, all tool calls collapse into a
"See reasoning (N steps)" disclosure. Click to expand and see what
the agent did. The final text answer stays visible.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/sidepanel.css

@@ -290,6 +290,32 @@ body::after {
   line-height: 1.5;
   word-break: break-word;
 }
+/* Collapsed reasoning disclosure */
+.agent-reasoning {
+  margin: 4px 0;
+}
+.agent-reasoning summary {
+  cursor: pointer;
+  font-size: 11px;
+  font-family: var(--font-mono);
+  color: var(--text-meta);
+  padding: 3px 0;
+  user-select: none;
+  list-style: none;
+}
+.agent-reasoning summary::before {
+  content: '▶ ';
+  font-size: 9px;
+}
+.agent-reasoning[open] summary::before {
+  content: '▼ ';
+}
+.agent-reasoning summary:hover {
+  color: var(--text-label);
+}
+.agent-reasoning .agent-tool {
+  margin-left: 4px;
+}
 /* Legacy classes kept for compat */
 .tool-name {
   color: var(--amber-500);

extension/sidepanel.js

@@ -196,8 +196,27 @@ function handleAgentEvent(entry) {
     if (thinking) thinking.remove();
     updateStopButton(false);
     stopFastPoll();
-    // Add timestamp
+    // Collapse tool calls into a "See reasoning" disclosure
     if (agentContainer) {
+      const tools = agentContainer.querySelectorAll('.agent-tool');
+      if (tools.length > 0) {
+        const details = document.createElement('details');
+        details.className = 'agent-reasoning';
+        const summary = document.createElement('summary');
+        summary.textContent = `See reasoning (${tools.length} step${tools.length > 1 ? 's' : ''})`;
+        details.appendChild(summary);
+        for (const tool of tools) {
+          details.appendChild(tool);
+        }
+        // Insert the disclosure before the text response (if any)
+        const textEl = agentContainer.querySelector('.agent-text');
+        if (textEl) {
+          agentContainer.insertBefore(details, textEl);
+        } else {
+          agentContainer.appendChild(details);
+        }
+      }
+      // Add timestamp
       const ts = document.createElement('span');
       ts.className = 'chat-time';
       ts.textContent = formatChatTime(entry.ts);