feat: add command output capture to troubleshooting sessions (#57)

Engineers can now paste command output during action steps. Output is
stored in the session decisions JSONB, displayed in session review,
included in all 4 export formats with command context, and preserved
in session-to-tree conversions.

- Collapsible "Paste Output" textarea on action nodes with commands
- 10,000 character limit with live character count
- Works on both built-in and custom action steps
- Preloads output when revisiting a step via Go Back
- All exports show commands run alongside captured output

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SessionDetailPage.tsx

@@ -224,6 +224,7 @@ export function SessionDetailPage() {
     if (decision.answer) lines.push(`Answer: ${decision.answer}`)
     if (decision.action_performed) lines.push(`Action: ${decision.action_performed}`)
     if (decision.notes) lines.push(`Notes: ${decision.notes}`)
+    if (decision.command_output) lines.push(`Output:\n${decision.command_output}`)
     try {
       await navigator.clipboard.writeText(lines.join('\n'))
       setCopiedStepIndex(index)
@@ -439,6 +440,14 @@ export function SessionDetailPage() {
                           Notes: {decision.notes}
                         </p>
                       )}
+                      {decision.command_output && (
+                        <div className="mt-2">
+                          <p className="mb-1 text-xs font-medium text-white/50">Command Output</p>
+                          <pre className="overflow-x-auto rounded bg-white/5 p-2 text-xs font-mono text-white/60 whitespace-pre-wrap">
+                            {decision.command_output}
+                          </pre>
+                        </div>
+                      )}
                       {decision.duration_seconds != null && (
                         <p className="mt-2 text-xs text-white/50">
                           Duration: {formatDuration(decision.duration_seconds)}