[UI] Response block rendering with left border + task live status

[randomm]

Problem

oclite UI has multiple visual issues compared to opencode TUI:

1. No visual separation — Agent text mixed with tool calls, no clear blocks
2. Task line missing live status — Should show → --- or current child tool output below task line
3. Inconsistent styling — Background color partially applied, truncated

Solution: Vertical Line Border (like TUI)

Use left vertical line (│) to distinguish agent output — user confirmed this approach is preferred.

Requirements

Left Border on Agent Output

• Add vertical line (│) on left side of agent prose/response text
• Use theme color (cyan or configurable)
• Tool calls and task lines can remain without border (they have icons)

Task Live Status Line

• Below each running task line, show: → [current child tool or ---]
• Update in real-time as child tools execute
• Clear when task completes

Remove Background Color Approach

• Remove the theme.prose.bg background color
• Use vertical line instead for visual distinction

Acceptance Criteria

• Agent prose text has left vertical border
• Task lines show live child status below (→ ---)
• Clean visual separation between user and agent
• No truncated/inconsistent background colors