fix: clarify scheduled task intent and summaries

[atomantic]

Summary

• Show a human-readable summary on every built-in scheduled-task card and in its configuration header.
• Explain hook-owned runtime prompts such as issue-watcher instead of displaying “No prompt configured”.
• Add explicit metadata and server-side guards for future automation-only task types, and clarify that pipeline stages are nested inside their parent task.
• Let custom task cards accept an optional one-line summary.

Validation

• cd server && npm test -- --run services/taskSchedule.test.js services/taskScheduleModules.test.js
• cd client && npm test -- --run src/components/cos/tabs/schedule/AppTaskCard.test.jsx src/components/cos/tabs/schedule/PromptEditor.test.jsx src/components/cos/tabs/schedule/GlobalConfigControls.test.jsx src/components/apps/tabs/CustomTasksSection.test.jsx src/components/cos/tabs/JobsTab.test.jsx
• cd client && npm run lint
• cd client && npm run build