Agent tasks can float above the conversation

A new floating task button and popover make active agent tasks easier to reach while you are reading or scrolling through a long transcript. The task UI is backed by explicit state and tests instead of being hidden inside the main agent view.

• Added KajiAgentFloatingTaskState
• Added KajiAgentFloatingTaskButton
• Updated KajiAgentHome, message rows, turns, and tool groups for the floating task flow
• Added KajiAgentFloatingTaskStateTests
• Documented the floating task and scroll-lock plan

Timeline updates are coalesced before hitting the UI

Agent transcript updates now pass through a coalescer, reducing noisy UI churn during streaming output and tool updates. Scroll lock handling was also hardened for stop-loader and transcript glitches.

• Added KajiAgentTimelineUpdateCoalescer
• Updated KajiAgentScrollCoordinator and KajiAgentScrollLockPolicy
• Improved tool timeline application behavior
• Added tests for update coalescing, scroll lock policy, abort reconciliation, and transcript virtualization
• Bumped Kaji to 0.2.6