Unified timeline view on task detail page

Replace separate Comments and Agent Log sections with a single unified timeline.

Display chronologically with icons per event type:

• 💬 comment
• 🔄 status_change
• 🤖 claim
• 🔧 tool_call
• 📄 tool_result (collapsible)
• 💭 thought
• 📍 checkpoint
• ⚠️ guardrail
• ❌ error
• ✅ complete

Show actor label: [human], [junior], [system] Show relative timestamp. Tool results should be collapsed by default.