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/schemas/session.py

@@ -28,6 +28,7 @@ class DecisionRecord(BaseModel):
     answer: Optional[str] = None
     action_performed: Optional[str] = None
     notes: Optional[str] = None
+    command_output: Optional[str] = Field(None, max_length=10000)
     automation_used: Optional[bool] = False
     timestamp: datetime
     entered_at: Optional[datetime] = None