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>

backend/app/core/session_to_tree.py

@@ -133,6 +133,10 @@ def _create_node_from_original(
         new_node["action"] = original_node.get("action", "")
         if decision and decision.get("action_performed"):
             new_node["action"] = decision["action_performed"]
+        if decision and decision.get("command_output"):
+            output = decision["command_output"].strip()
+            if output:
+                new_node["action"] += f"\n\nCommand Output:\n{output}"
     elif node_type == "solution":
         new_node["solution"] = original_node.get("solution", "")