feat(sisyphus-task): add skillContent support to background agent launching

- Add optional skillContent field to LaunchInput type
- Implement buildSystemContent utility to combine skill and category prompts
- Update BackgroundManager to pass skillContent as system parameter
- Add comprehensive tests for skillContent optionality and buildSystemContent logic

🤖 Generated with assistance of oh-my-opencode

src/features/background-agent/manager.ts

@@ -140,6 +140,7 @@ export class BackgroundManager {
       path: { id: sessionID },
       body: {
         agent: input.agent,
+        system: input.skillContent,
         tools: {
           task: false,
           call_omo_agent: false,