feat: add per-agent timeline tab to agent inspector

[DorianZheng]

Summary

• add a new Timeline tab to the agent inspector with reverse-chronological agent history
• merge workflow item events for the agent with agent activity events, with expandable cards, type/date filters, and markdown export to clipboard
• keep the existing inspector content under the Overview tab

Files Changed

• src/renderer/features/agents/components/AgentContextPanel.tsx
• src/renderer/features/agents/components/AgentTimeline.tsx

How to Test

• open an agent inspector and switch between Overview and Timeline
• confirm workflow events and activity events are merged newest-first for that agent
• filter by event type and date range, then expand entries to inspect full detail
• click Export as Markdown and confirm the generated report is copied to the clipboard