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/types/session.ts

@@ -9,6 +9,7 @@ export interface DecisionRecord {
   answer: string | null
   action_performed: string | null
   notes: string | null
+  command_output?: string | null
   automation_used: boolean
   timestamp: string
   entered_at?: string | null