feat: cross-layer interaction — hover highlight, j/k auto-pan, agent step jump

[lis186]

Part of #91

Sub-items

1. Hover cross-highlighting

Hovering swimlane bar → cursor in overview + step scrollIntoView.
Hovering step → cursor in swimlane.
150ms debounce.

2. j/k auto-pan swimlane viewport

When j/k navigates to a turn outside viewT0..viewT1, auto-pan viewport to center on the selected turn.

3. Agent tool call → subagent lane jump

Step list renders Agent tool calls with description. Add ⤴ button that time-window-matches to the corresponding subagent lane and switches wfState.selectedLane.

Data limitation: Agent tool_use input has name/description/prompt but no subagent session ID. Time-window matching (parent turn receivedAt..+elapsed overlapping child entry start) is the current-best approach.